2001 CRV vibration noise

Folks,
My old reliable CRV started making a loud vibration, like the tailpipe
is rattling at 2300 rpm's and 53 miles per hour. It's a quick
virbration that disappears above and below that speed but is very loud
when it happens. The engine has to be pulling fairly hard.

It sometimes occurs at 2300 in lower gears but not as often.

The tailpipe and muffler look solidly mounted and the hangers are in
good shape. Any ideas where it can be coming from? It sounds like
under the rear portion of the vehicle.

Thank You