I have this very annoying drone/vibration at 2200RPM. It sounds through the exhaust, but hard to nail down. It's in drive/driving and (a very slight lesser degree) in park too. It reminds me of a driveshaft that's out of balance/pinion angle off. That type of on/off drone. The car runs/drives excellent. In fact best it ever has, except this drone. Goes away as soon as it goes above/below that rpm. I'm pulling my already bald head hair out!

...It's a stroked 400/727/8.75 and a 67 Coronet.