thumping sound

When I crank my 93 vr4 and let it warm up, if I get behind the car near the muffler i hear a thumping sound. Its as if I have a subwoofer on in my car, but its coming from the muffler. Its kinda like a backfire but just a thump. no cel comes on? been doing this since i bought it

does it backfire at all when you rev it up?

If it's just happens every so often at idle I wouldn't worry about it... Every engine misfires a certain percentage of the ignition... That is actually why all cat converters are 3 stage and how the bulk of the hydrocarbons end up on the exhaust...

You can help to eliminating it (in no particular order) by:
1) playing with your spark plug gap
2) buying better wires
3) buying new injectors
4) buying an aftermarket multi-spark ignition system

But even then it potentially will not eliminate the "thump"... It really depends on how bad the misfire is...