Stop in for a cup of coffee

So Im taking Dart to My exhaust guy, Since I got OD to work, the cruise RPM causes a drone that is unbearable!! Flowmaster 3 chamber mufflers, 2.5 pipe, H pipe! Something needs to be done... Turn off OD, drone is gone, different RPM...

Hmm.... Mounts/hangars maybe? Or do you think it's just muffler tone?