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In a 2004 VW R32, after shutting down the engine, a repeated thump is heard approximately 2 times a second from the rear of the car.

What are the likely sources of this noise? My first thought is something related to the evaporative emissions (EVAP) system.

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My cars both have a buzzing sound from the charcoal canister (sounds like a little electric motor running, but is just escaping air), so it's possible. – Brian Knoblauch Nov 1 '12 at 20:11
Did you check the trunk for stowaways? – Tim Nov 2 '12 at 14:51
I have a similar issue with a 2008 GTI. I was thinking fuel/emissions related. It sounds like a motor/pump of some sort? Mine does this while driving sometimes (and it's not the fuel pump, AFAICT). – Nick Nov 5 '12 at 2:18

It is most likely a leaking EVAP hose or a leak in the fuel line. Make sure it's nothing obvious like a hose hanging off or a gas cap missing. Also check for a check engine light and have the OBD code read if there is one.

There is also a small chance that your fuel pump relay or fuel pump is the cause.

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