Ok, today going to my friends shop I pressed the gas and nothing happened but a light thunk! I think it came from the exhaust. It's done it before but a long time ago. What could cause this?

 

Need more info to accurately diagnose.
Only thing I can think of now is exhaust leak or ignition problems.

 

I'm thinking it might be the ignition coil because it's just a $15 one from Advance. Then again I have heard it could be the TPS. Aren't those known for being a pain in a few different ways? The car has high miles and alot these sensors seem to be the original ones. Thinking about replacing it just to know there is a new one.