Front insulator/right mtr mount noise

[Dan Mitchell]

I have the groaning noise on torque reaction which I gather from reading the
sites is due to a failed front insulator/right mtr mount. Seems easily
enough replaced, but I'm wondering exactly what is MAKING the SOUND.  Sounds
kinda like an exhaust leak, but not really.  Is it the mount itself (hard to
imagine) suspension bushings, or what?  Mine does it 1)in reverse 2)going
over dips  3) when dropping the clutch suddenly while coasting in low gear.

Just curious...
Dan