What is this noise?
 

Hi everyone - I have been having this noise of shutoff for about the last 8-10 months. I was finally aware enough and able to capture it today with my phone....It doesn't happen every time, maybe about 70% of the time. It is from underneath the car by the rear passenger-side area. First there is a slight knocking followed by some type of decompression - hopefully it is clearly audible in the attached WMV.

3.0i 2005 with about 229K miles - no rear air suspension.

Thank you!
Nick

I can't think of any "compression" noises from the back other than air bags, compressor or tyres. The knocking is of course the result of the loss of compression. Have you tried lying under the car to listen/see...maybe that will pinpoint the location of the noise?

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Dies your X have rear air suspension?

I should have mentioned originally that I do not have rear air suspension, so that certainly eliminates one potential cause.

I have tried to crawl under the car a bit and pinpoint, but this sound clip is really the best I could do. I think it sounds like it's coming from slightly in front of the rear passenger tire area, and about 1-2 feet towards the center of the vehicle (underneath) - I'm sorry I can't provide a more accurate description than that.

Since you dont have air suspension I can't think of any other pressure line/reservoir or any of the sort in the location you're describing. Perhaps brake lines but you'd get a warning in the dash for that. I'm baffled.

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Maybe exhaust rubbing on something as it's cooling and contracting ??

I'm unsure if that's possible - it has happened both after decent drives (20-30 minutes+) and also after I have just had the car idling for 5 minutes after a cold start and then shut off - in the latter case I'm unsure if the exhaust would even have a chance to really heat up....

Maybe get yourself a piece of hose that you can hold to your ear and prob around with the other end when it's happening to try and pin point it. It's good that it happens often because it will be much easier to pin down.

just throwing this out -
we have a "vapor leak detection pump" as part of the fuel system. Maybe after shut off it empties and yours is getting noisy??
I believe its under/close the tank (under back seat).

Certainly sounds plausible - no check engine light though, so it's still working OK I guess. I'll have to investigate it more, thanks!