Hi there,
I'm not familiar with the 420, but does it have a center bearing on the driveshaft? It sounds like a dry center bearing to me... if it has one, get one of those long skinny needle grease tips and go under the bearing seal to grease it and see if the noise goes away. If it does, replace the center bearing! I had a Datsun pickup once that exhibited the same exact symptom, and I replaced the transmission before wising up and lubing the center bearing! At least I replaced a 4 spd with a 5 speed.

Good luck,
Richard Wooldridge
1982 300D/4.3LV6
1994 Ford Taurus (wife's car)
1988 Astrovan
1977 Jag XJ6L
Etc...