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There is an occasional ticking noise that I can distinctly hear right behind my head while at idle. It's not connected to engine speed and will speed up and slow down at it's own will. Then will disappear completely. It sound like it is in the top of the engine compartment right behind the driver side seat.

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I spent quite a bit of time listening today with the engine bay open. This noise does, in fact, tie to engine speed. It does, however, either disappear or become drowned out by other engine noise as the revs get up 1300 rpm. If I rev the engine above that and then slowly bring it back down I can hear it in sync with the engine. However, if I rev the engine and then let off the throttle, letting the revs drop quickly, it does not show back up until the engine has idled for a split second. Then it starts back slowly (somewhat out of sync with the engine) and quickly speeds up to engine speed. It never starts until the engine is warmed up to operating temprature. Also, it occasionally comes and goes. During the 15-20 minutes I let the engine run to listen, it stopped while at idle twice for at least 1 minute but not more than 3.

It sounds like it is coming from the driver side rear of the engine compartment and it also sounds like it reverberates through the intake manifold. However, I know sound can be really deceiving. I'm confident it is not the water pump as I can hear the sound much less laying on the ground behind the passenger side door. (I believe the water pump is on the lower, passenger side of the engine.)

Could this possibly be a vacuum leak? I do have a CEL light that the error codes were P1128, P1130 and P1115, but I figured it was just a dirty MAF and have ordered the tools to clean it.

Any help would greatly be appreciated. I'm not exactly a rookie, but certainly have tons to learn. I am a Boxster rookie, however.

Oh, by the way, 2000 Boxster, manual transmission, 99.5k miles.

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P1128 Oxygen Sensing Adaptation, Idle Range, Bank 1 – Below Limit

P1130 Oxygen Sensing Adaptation, Idle Range, Bank 2 – Below Limit

Potential causes:

– Incorrect signal from MAF sensor

– Fuel pressure too high

– Fuel injector leaking

– EVAP canister purge valve open

P1115 Oxygen Sensor Heating Ahead of TWC, Bank 1 – Above Limit

P1115 Oxygen Sensor Heating Ahead of TWC, Bank 1 – Below Limit

P1115 Oxygen Sensor Heating Ahead of TWC, Bank 1 – Open Circuit

P1115 Oxygen Sensor Heating Ahead of TWC, Bank 1 – Signal Implausible

You need more investigation here.

Potential causes (depending on further investigation):

– Heating resistance too low

– Short to B+

– Corrosion in connector

– Break in wiring except for a few litz wires

– Heating resistance too great

– Oxygen sensor

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Thanks Loren. I'll definitely check into the O2 Sensor issue further. Unfortunately, as I suspected, it's highly unlikely that has anything to do with what I am hearing. ARGH!

I had a similar problem on a BMW & it turned out to be a cracked weld between one of the pipes & the flange of the exhaust manifold

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Thanks Loren. I'll definitely check into the O2 Sensor issue further. Unfortunately, as I suspected, it's highly unlikely that has anything to do with what I am hearing. ARGH!

I had a similar problem on a BMW & it turned out to be a cracked weld between one of the pipes & the flange of the exhaust manifold

Hmmm.... that is possible. I'll look for cracks/leaks in the exhaust. That could possibly make a noise like this as well. I think. Either way, thanks for the idea.

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Thanks to those who weighed in here. It turned out to be a rattling catalytic converter. Granted it doesn't sound like any cat rattle I have ever heard before, however that's what it is. Does anyone know if I can replace the headers all together (removing the first set of cats) and get the O2 sensor to behave?

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