Elantra vibration noise

I have a 2001 Elantra VE. It has started running noisier than it used
to. I feel vibrations through the interior at certain engine speeds,
and it is somewhat noisier outside the car too. It is still running
smoothly otherwise -- no hesitations or anything like that. The
vibration started fairly suddenly between two trips on the same day.

Given that the engine is still running smoothly, I'm thinking that it's
most likely a hole in the exhaust somewhere. I would expect more
external noise in that case, however. Is there something (else) I
should be looking for?

Thanks.

 

It's possible one of the exhaust heat shields (or something else attached
to the engine) is loose that has a resonant frequency equal to the engine
frequency at the rpm's when the issue occurs.