your assistant place a rag over the exhaust while the engine
is running. (Note they are not trying to block the exhaust
just impede it)
Check system for leaks.
This will be a lot easier with the car jacked up but if you
cannot jack the car up, a leak should still be more obvious
when the pipe is partially blocked with a rag.
Joints are the normal places for leaks to occur and can sometimes
be fixed by simply tightening up the bolts that hold the joints