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Jack_H

Knock-knock sound from right side of fuel tank

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Hi,

i have a  knock-knock sound coming from under my 2008 Cayenne S,  towards the rear (in front of the right rear wheel). It sounds like it is coming from inside the fuel tank.

i am not getting a check engine light and the car starts and runs as normal. The knocking sound begins about 3 seconds after the car comes to a stop and begins to idle. If I increase the engine revs, the frequency of the knock-knock sound does not change.

 

Note that the sound is definitely not from the air compressor.

 

Where could this noise be coming from? Thanks in advance for any ideas.

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Posted (edited)

Thanks Loren.

i checked the exhaust system/muffler mounts and all looks good. The sound seems to be coming from somewhere inside.

Edited by Jack_H

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Thanks Loren. After some more research, I thought of pulling the fuel pump fuses to see if the noise is coming from either of the two fuel pumps. It’s not.  I pulled fuse 13 first, the noise remained and the car stalled after about 15 seconds. I put fuse 13 back in, started the car, waited for the sound to start again which it did after idling for about a minute, then I pulled out fuse 14. The engine kept running but the sound still remained.

Listening to it some more today, it sounds like a vacuum valve or some sort of a vacuum related sound. My car does not have the exhaust/muffler valves that activate in sports mode. Is there any other vacuum valve or something similar in the tank?

 

 

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*** update ***

after writing my previous post, I pulled fuse number 8 (in engine bay) and the noise stopped.  Put the fuse back in and the annoying sound returned. Out again and the sound stopped. Fuse 8 controls ‘Tank vent valve “ and a few other things. I guess I have a faulty tank vent valve. Now I need to figure out how to get to it and replace it.....

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