metallic noise

[belleriver]

2003 996 C2 cab 6spd 84,000km

I have this oscillating metallic noise during idle at startup coming from the engine compartment: "shing" - 1 second pause - "shing" - 1 second pause - "shing" etc... it almost sounds like a knife blade against a metal sharpening tool, very high pitched. As soon as the rpm's drop off it makes a sound like a fan being stopped and disappears.

The only other time I heard it was when I got off the highway after about 2 hours of driving and was idling at a light, it was a lot louder and again sounded like an out of balabce fan hitting a piece of metal, then it stopped while I was driving. The A/C on/off does not seem to be tied to it.

I searched the forum but did not see anything.

Anyone have any idea what it could be?

[RFM]

Remains the same sound with clutch depressed? Has been tried with the poly V belt removed? When the noise is gone than, check all the pulleys.

[wvicary]

Secondary Air Injection Pump might also be at fault. They cycle during the warm up phase but under certain conditions can run a shorter cycle when the engine is at operating temp. When the belt is off pay careful attention to the water pump.

[belleriver]

Thanks for the input.

I normally hear the sound as soon as I start the engine. The car is not in gear and I have not moved yet. I will see if depressing the clutch changes anything as suggested before I do anything with the poly belt.

Regarding secondary air injection pump, what does this do? Over the last few weeks I've noticed that when I take my foot off the gas with the clutch still engaged I do not get a nice smooth deceleration, its more of an uneven/staggered deceleration - could this be related?

[belleriver]

Clutch up or down - same noise. I will pull the belt this weekend to check all the pulleys. I assume I am ok to spin each one by hand trying to get the noise to repeat. Just to double check, can anyone confirm there is no reason I should not do this to any of the components driven by the belt?

Also, can anyone please tell me what the 1-8 items are in the attached diagram for reference.

[wvicary]

You will not damage anything by spinning it by hand.

1- water pump

2- alternator

3-idler pulley

4- P/S pump

5- A/C compressor

6- engine crankshaft drive pulley

7- belt tensioner pulley

8- idler pulley

[belleriver]

Checked it out on the weekend. Its definately my air pump making the sound. I pulled the pump and disassembled it but could not find anything obviously wrong with it. Plugged it back in, pulled the outlet hose with the engine running and it is pushing plenty of air. Maybe one of the blades is somehow out of balance? $500 new. Not sure if I'm going to replace it just yet.

[logray]

The last time I checked air pumps were more in the $1000-$1500 USD range.

Nice work isolating the problem.

You can get a used one from a breaker, with a limited warranty for $300 USD.

http://dcauto.gotdns.com/search/index?target=99660510400&searcher=&commit=Search

[RFM]

belleriver, if i were you i would take a second look, you write in your first post literally, and i quote, "the only other time i heard it was when i got off the highway after about 2 hours of driving" The air pump cant run under this circumstance - hot engine - unless there is something wrong with the management systems, a PIWIS diagnostic tool is needed in that case.

[Loren]

RFM I have seen this also. The air pump will turn on for maybe 5 or 10 seconds then turn off again after sustained driving at speed and then coming to an idle at an exit.

[belleriver]

I'm very confident (but realize I could be wrong) that it is the air pump as I was able to put my hand on it at the momoent it was making the noise during idle and I could feel the vibration was in sync with the noise. I kept my hand on it until the pump came to a stop. While the pump was winding down you could hear the noise get louder, the frequency reduce and again the vibration was completely in sync with the noise. Noise and vibration stopped as the pump came to rest.

The pump is made up of 2 fan blades anchored to the rotating shaft. There is a third blade positioned between them mounted to a plastic plate that sits inside the main housing (press fit in place). It appears this middle blade is intended to remain stationary while the others rotate on either side. I could not see anything that could generate this sound outside of the middle (third) fan blade creeping out of position and rubbing agains the moving blades. I put some tape on the the plastic part of the middle fan blade and the housing to help hold it in place. I am not 100% sure if this made a difference yet as the noise was intermittent. I will monitor it for a few more days and report back.

The pump I found was $425 and appears to be the OEM part (Bosch)

http://parts.autopartsonlinecanada.com/parts/apocanada/wizard.jsp?year=2003&make=PO&model=911--011&category=B∂=Air+Pump

Thanks for all the input. Great forum!!

[RFM]

Sorry for the wrong info in this case, over here the pump only run when the management put the system in open loop mode, for example when the O² sensors are not warmed up, cold engine, etc. to dilute in such circumstances the too rich exhaust emissions. Perhaps a difference in local legislation.

[logray]

I remember reading there might be a VW or Audi part that is a direct replacement, might have been from Orient Express.