I switched my 1992 Rs from its stock 2" exhaust to a 3" pipe all the way back. since i made the switch i can hear a "thump" every time i hit a bump in the road and i cannot find where it is hitting. Its very annoying, but i dont think its bad to drive it. Is this problem common?

 

The exhaust is not the problem, the installation is the cause.

 

You have some pipe to body/suspension clearance issues.
And 3" pipes are way overkill for your 305. It's enough actually to hurt your low end torque, which is the only thing the 305 has to offer anyway.

 

When is a "Thump" ok to drive on? Eventually that thump will lead to your new 3" system falling out and tearing everything in its way along with it. Find your thump and fix it. You need to anchor the exhaust somewhere, as it sounds like your hanger brackets are just letting it flop around.

 

your right 69 buy a 350 to fit your 3'' pipes lol no but if u can shake any part of your system u need to fix it. if ur system is good it could be clearence from ur rear. get it fix now before "it falls out n takes everything with it"