GM Service Manual - Driveline Vib Diagnosis

[wilkin250r]

I definitely have a "boom/beat" problem in a 72 I'm fixing up. The vibration is fairly high frequency, so I'm assuming it's a driveline issue and not a tire issue, but the vibration also fades in and out with a regular rhythm.

Am I correct in assuming the transmission HAS to be part of the problem? Exactly like you said, Keith, amplitudes adding (in phase) or canceling (out of phase). With a driveline imbalance or u-joint, it would be a constant vibration. It wouldn't fade in and out because there is nothing to change. The planetary gears in the transmission are the only thing I can think that would allow two similar vibrations to fade in and out of phase with each other, so one of the vibrations HAS to be in the transmission. Yes?

(Actually, as I type this, I suppose the ring/pinion gears could also allow two vibrations to add/cancel each other)