Knock-knock sound from right side of fuel tank

[Jack_H]

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.

[Loren]

 

Check for a loose or broken muffler/tailpipe mount on that side of the car.

[Jack_H]

Thanks Loren.

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

Edited May 13, 2019 by Jack_H

[Loren]

Well, the only thing inside the fuel tank that could make noise is the fuel pumps.

[Jack_H]

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?

 

 

[Jack_H]

*** 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.....

[Jack_H]

**update #2**

 

I changed the tank vent valve and it did not make a difference. The annoying sound remained. Continued to drive the car until this weekend when I decided to have a look at the fuel pumps. When I took the back seat out and removed the cover of the fuel pump on the right hand side of the car, I found what looks like a vacuum or vent pipe that is not connected to anything. See attached picture. I had a good look around and could not see any other pipe that this is supposed to connect to. I started the car and when the tank vent valve activated, it began to pull a vacuum on this pipe which in turn started to make the "puck-puck-puck" sound, at a frequency of about two puck-pucks per second. When I put my finger on and closed it, I could feel it pulling a vacuum and the sound stopped.

I now put a cover on this pipe and blanked it off and the annoying sound is gone, but I am wondering what it's for and where is it supposed to be connected. Based on the date stamp on the top cover of the fuel pump, it does not look like it has been changed.

Any ideas what this pipe is for and what it is supposed to be connected to?

[Tecchie]

This is interesting, I'd like to know where this goes also, just in case I run into this issue at a later date..

[Rick150]

The connection is there.