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Engine Noise - buzzing or sheet metal vibrating


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Need some help from the experts.

Car:

  • 2.5 '99 Tip, 82K miles
    no CEL
    runs fine, decent acceleration (except when noisy - could just be paranoia)

Service Record:

  • new engine around 60K miles because of RMS failure
    recent oil change w/ Mobil 0W40
    new air filter
    precat O2 sensors 5K miles old
    new post cat O2 sensors
    new spark plugs
    MAF 5K miles old
    new AOS
    cleaned throttle body, ICV, intake manifolds (areas that I could reach)
    new polyrib belt
    rotors and pads 10K miles old

Problem:

  • A buzzing or grinding noise between 1000 and 25000 rpm
    sound seems to come from driver side intake manifold as if something inside is vibrating
    happens only intermittently from day to day (if noisy one day then will be noisy all day, if no buzz one day then quiet all day)
    happens when I'm trying to keep a certain speed on highway; ease off on accel then lightly depress to maintain speed
    not as noisy when accelerating hard
    happens when in gear or in park

What it isn't:

  • not loose spark plugs (tightened to 21 ft-lbs with torque wrench)
    not leaky intake (no engine surge with carb cleaner or propane spray)
    not pulleys or polyrib belt (ran car briefly w/o belt and no noise, spun pulleys by hand and looks true)
    doesn't seem to be any loose body panel, heat shield, etc.
    not AC compressor (noisy w/o AC on)

What I'm thinking:

  • replace fuel filter
    check fuel pressure (having hard time finding Shrader valve with correct threads)
    clean injectors (how? aside from running a bottle of Techron in tank)
    compression test
    maybe replace water pump (but wp is on opposite side of engine where noise is coming from)
    alternator? (same thing - it is not on same side as noise)
    could a failing power steering pump do this?

If you have ANY ideas or suggestions please help. It is driving me crazy!

Thanks, Hung

Edited by pomocanthus
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  • Moderators

You always can check the power steering level and the possition - state of the front engine mount, the silent block have to sit straight in the middle of the mount and free of cracks.

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Need some help from the experts.

Car:

  • 2.5 '99 Tip, 82K miles
    no CEL
    runs fine, decent acceleration (except when noisy - could just be paranoia)

Service Record:

  • new engine around 60K miles because of RMS failure

recent oil change w/ Mobil 0W40

...If you have ANY ideas or suggestions please help. It is driving me crazy!

Thanks, Hung

Don’t think it's your problem but the Schrader valve is on the passenger side fuel rail up near the cockpit. Looks like a bolt sticking straight up. About a 13 mm.

Doubt it's the prob but you could also check your snorkle and opening (unlikely to "grind but could rattle....)

If you can get it to buzz with car unloded and parked, a good ole trick is to but a piece of fuel line in your ear and probe around while you hold the throttle at the pond of resonance. Like stethoscope.

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It is extremely common for heat shields to loosen up and cause an awful racket. The circular ones near the cats will loosen up sometimes and rattle like mad and other times be completely silent. Your description is very consistent in many ways with this kind of problem (except the part where you say it's not a heat shield :) )

To diagnose a heat shield, get under the car with a rubber mallet and hit the exhaust with it. Heat shields will usually rattle given that sort of provocation.

I wouldn't get fancy with diagnosis until you've tried this.

---

It also could be a bearing in one of the pulleys that the PolyRib belt turns. For example, does it happen more if the AC (snowflake icon) is on?

You can remove the engine panel behind the seats and drive the car and see if you can get the sound to happen.

But seriously, I would check the heat shields first and foremost.

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It is extremely common for heat shields to loosen up and cause an awful racket. The circular ones near the cats will loosen up sometimes and rattle like mad and other times be completely silent. Your description is very consistent in many ways with this kind of problem (except the part where you say it's not a heat shield :) )

To diagnose a heat shield, get under the car with a rubber mallet and hit the exhaust with it. Heat shields will usually rattle given that sort of provocation.

I wouldn't get fancy with diagnosis until you've tried this.

---

It also could be a bearing in one of the pulleys that the PolyRib belt turns. For example, does it happen more if the AC (snowflake icon) is on?

You can remove the engine panel behind the seats and drive the car and see if you can get the sound to happen.

But seriously, I would check the heat shields first and foremost.

Thanks Stefan. I will give that a try. The only reason I concluded that it wasn't the heat shields is because of the apparent dependency on the RPM range (BTW I don't have a super Boxster, that is 2500 not 25000 RPM) and how I have to feather the accel to get the noise. I guess it is possible for the loose heat shield to have resonant frequency near that of engine at these RPMs. Do I need to remove the post cat O2 sensors before banging on the muffler? I've heard that they are sensitive to shock.

Will report back with solution (hopefully)

Hung

Edited by pomocanthus
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It is extremely common for heat shields to loosen up and cause an awful racket. The circular ones near the cats will loosen up sometimes and rattle like mad and other times be completely silent. Your description is very consistent in many ways with this kind of problem (except the part where you say it's not a heat shield :) )

To diagnose a heat shield, get under the car with a rubber mallet and hit the exhaust with it. Heat shields will usually rattle given that sort of provocation.

I wouldn't get fancy with diagnosis until you've tried this.

---

It also could be a bearing in one of the pulleys that the PolyRib belt turns. For example, does it happen more if the AC (snowflake icon) is on?

You can remove the engine panel behind the seats and drive the car and see if you can get the sound to happen.

But seriously, I would check the heat shields first and foremost.

Thanks Stefan. I will give that a try. The only reason I concluded that it wasn't the heat shields is because of the apparent dependency on the RPM range (BTW I don't have a super Boxster, that is 2500 not 25000 RPM) and how I have to feather the accel to get the noise. I guess it is possible for the loose heat shield to have resonant frequency near that of engine at these RPMs. Do I need to remove the post cat O2 sensors before banging on the muffler? I've heard that they are sensitive to shock.

Will report back with solution (hopefully)

Hung

I have a '99 Boxster and my heat shields rattle and the problem you describe is exactly what's going on with my car. I'll be mounting them up good and tight this weekend.

Edited by benspeeder
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Thanks Stefan. I will give that a try. The only reason I concluded that it wasn't the heat shields is because of the apparent dependency on the RPM range (BTW I don't have a super Boxster, that is 2500 not 25000 RPM) and how I have to feather the accel to get the noise. I guess it is possible for the loose heat shield to have resonant frequency near that of engine at these RPMs. Do I need to remove the post cat O2 sensors before banging on the muffler? I've heard that they are sensitive to shock.

Will report back with solution (hopefully)

Hung

Hung:

I agree with Stefan. You may have it the nail on the head because resonance can definitely cause the shields to vibrate. It's happened to me.

Regards, Maurice.

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Thanks to everyone for the inputs. It's been rainy and cold the last few days so I haven't been able to try anything with the car. Coincidentally, she has been purring like a kitten w/o a sign of heat shield resonance (I'll coin the term HSR). Aside from heat shield clamps and fasteners, are there other means for keeping them from vibrating? I'm thinking there are probably high temperature silicone or foam that I can use to dampen whatever is loose. I'll look this up and report if I find something easy, cheap, and available ... (insert your joke here) :P

Sorry I couldn't help that one.

Hung

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Aside from heat shield clamps and fasteners, are there other means for keeping them from vibrating?

the heat shields rattle because the fasteners rust out. you can either rip the heat shields out completely (which is what i did. no problems for over 4 years now) or buy some fasteners that won't rust (Home Depot, Lowe's, etc.)

in the diagram, it looks like parts 25, 26 and 27 are what you need. the sizes are listed at this link http://www.pelicanparts.com/PartsLookup/HT...2-00-Frame3.htm when you click on the part. note the sizes you need and buy some parts at your local hardware store that won't rust. or maybe buy the parts from Porsche but spray them with a high-heat rust protectant.

986_USA_KATALOG-063.jpg

Edited by Chris_in_NH
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There are two relevant heat shields. There are the rounded sheet ones that go around the cats. Those ones get loose because the fasteners corrode, as Chris in NH pointed out.

There are also heat shields that wrap around the exhaust pipe in front of the catalytic converter. These are harder to fix. I believe they are spot welded onto the pipes. When the welds break, the shields start to rattle. You can bend the metal to get them to be tighter but they will eventually start rattling again. The correct solution is to reweld them. I removed them and replaced them with heat wrap.

Incidentally, when heat shields rattle it is very often only within a particular rev range.

When you use a mallet to bang on the exhaust, you should not have to hit it very hard and you should use a rubber mallet. So you shouldn't have to worry about the O2 sensors.

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  • 2 weeks later...

I just wanted to close out the topic and thank everyone for their help and advice. I replaced the headers with after-market ones and haven't heard the rattle since. Although it wasn't the old headers causing the rattle, in the process of banging out the pressed-in bolts on the cats for replacement I must have somehow reseated the loose heat shield(s). Now my car is noise free and accelerating much smoother.

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I just wanted to close out the topic and thank everyone for their help and advice. I replaced the headers with after-market ones and haven't heard the rattle since. Although it wasn't the old headers causing the rattle, in the process of banging out the pressed-in bolts on the cats for replacement I must have somehow reseated the loose heat shield(s). Now my car is noise free and accelerating much smoother.

Which headers did you go for?

Have you noticed any difference in peformance?

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I just wanted to close out the topic and thank everyone for their help and advice. I replaced the headers with after-market ones and haven't heard the rattle since. Although it wasn't the old headers causing the rattle, in the process of banging out the pressed-in bolts on the cats for replacement I must have somehow reseated the loose heat shield(s). Now my car is noise free and accelerating much smoother.

Which headers did you go for?

Have you noticed any difference in peformance?

atm,

I bought mine on eBay for around $130. I figured what the heck, it's cheap enough that if it didn't work out then I don't eat lunch for 3 days and spend that time putting back the factory headers. It turned out that I noticed a difference in the response of the car. Before, it seemed almost as if the engine couldn't push the exhaust out and now there's a whooosh sound every time I put my right foot down. Here's a link with differing opinions so you can make up your own mind. I for one had a very positive experience. I bought from "surewin" for what it's worth. I am not affiliated with this person in any way. You can probably still see my feedback.

Hung

http://www.renntech.org/forums/index.php?showtopic=17862

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