this is a total mystery?

[RobertTate]

i can set in my car with it running and after about 4 or 5 minutes theres a deep but not real loud thump that comes from the back of the car....im also gettin 215 miles to a full tank of gas even though i changed my fuel filter.

 

[sdpatt]

Does the thump sound like a low pitched rumbling that continues for more than a few seconds or is it just one thump? If it is the runbling you are hearing, It is the fuel return line bubbling fuel vapor up through the fuel in the tank. This can be caused by a worn fuel return line check valve that is clipped on the end of the return line at the fuel pump and sender assembly. The part is fairly inexpensive (check at auto supply stores first), but requires dropping the tank and pulling the fuel pump and sender assembly to access it.

 

[RobertTate]

no its just a thump thats ends as quick as it started

 

[pjtoledo]

At about the same time as the Thump, loosen the gas cap and see if the tank is pressurized, or under vacuum. You may have a problem with the vapor recovery system. I had a situation where the vapor line restrictor was clogged, resulting in a pressurized gas tank. Do you have a very trusting friend that is willing to lie under the car and listen for the "Thump"?? No doubt a true test of friendship, an alternate would be a mother-in-law.
Perry Toledo,Ohio

Or, it could just be the exhaust pipe expanding and shifting.