I'd check the entire exhaust system routing. I'd place strong odds on the exhaust system having rotted out somewhere along it's length, and the pipe is now resting against something and resonating.
Thanks, good idea.
A baffle in a muffler can be loose as well as the inner guts of a cat converter. Idler/ belt tensioner may be worn (accessory or timing belt). Loosen/remove accessory belt and bring it up to the RPM that the vibration is happening. Some vehicles are engineered such that the torque converter can be separated from the flex plate so that can be eliminated as a source for the vibration. Sometimes vibrations are tough to isolate....
Good idea with the belt/tensioner pulley.
The vehicle does make a squeaking noise when started up but eventually goes away.
I traced the squeaking to the belt/tensioner pulley, which I will come around to replacing.
But I don't really know if this noise is related.