Ok guys...Ive posted about this in the past and all i get are smart remarks...so lets try again. When my car is sitting(cold) during the night,...it makes a *thump* noise..it only happens every once in a while.the car has a new engine in it......the noise is coming from the back,this has been happening since ive owned the car for 4 yrs now,so obviously its not hurting it...its just really weird.Any ideas

 

when i say the noise is coming from the back..i mean towards the back of the car..not of the engine.
~darbster~

 

I think a thump comes from gastank.When outsidetemperature changes at cold night's ,pressure inside at tank moves down and causes move at gastank wall's.

 

I dunno, maybe your relief valves for gas pressure are clogged so much, that the pressure just blows up inside the tank, and makes thud. Either that or someone is trying to open the secret latch from the underground railroad, and they keep hitting your car.. let them breathe man, move your car!

 

He he thats funny.....so it could be a clogged relief valve...how could i fix/replace that if that is the problem?

 

Check the vent line and hose to the evap canister. The gas cap is not supposed to vent to the outside. Sounds like possibly when the gas cools down enough the tank draws down the same as a gas can would it the caps are on. If the line is OK and the hose to the canister is not plugged, the check valve in the canister may be bad. Remove the gas cap and try blowing air back to the tank while having someone listen at the filler neck. If the tank has ever been out, the line may have been kinked or pinched when the tank was reinstalled.