Ive come across a very strange problem with my 03 Evo. After about 2-3 seconds from shutting the engine off, there is a loud thump that comes from which seems to be the fuel tank. Once in a while I can hear/feel the thump while the car is running as well.

My car is bone stock minus a exhaust and FMIC. Any ideas?

 

the only thing I can say is take the back seat out and pull out the pump. Visually inspect it. Maybe you fuel pump is going bad.

 

It sounds like a venting problem to me. I had the problem on another car. As the fuel is pulled out it needs to be replaced by air or your gas tank is going to get smaller.

Try this test - as soon as the engine is off, open the filler cap and see what happens.