I have this s40 where when you start the car, you can hear a thumping noise coming from the gas tank. and it will continue for 20 sec after you shut it down. If you take the gas cap off, alot of pressure comes from the tank. I think the problem is with the wiring going to the diagnostic pump. If i disconnect the hose coming from the tank to the pump, I do not have the issue. I tried another known good pump and the same thing happens. So now the next step is to check the wiring. Just wondering if anyone has seen this before that can give me some tips.