71 Dart 727 w/suregrip having weird issue

My son and I are gradually getting rid of vibrations and rattles on his '65 Cuda. The exhaust was hung from small rubber blocks wrapped in steel and bolted to the chassis. No much shock or vibration absorption there. We replaced the rear hangers and it is better, and will replace the ones in the axle tunnel soon. We're using old fashioned rubber strap hangers now. It got worse in left turns and at certain RPM's so that was a pretty good sign of exhaust vibrations. And uphill at 75 MPH, it phases in and out with a driveline or wheel vibrations.

So sometimes you just gotta keep changing things; one can mask another.