Trying to sort out M46 noises

I drove my wagon a lot over the weekend helping some folks move and noticed a weird behavior from my car in 3rd. There are plenty of drones and rattles (40 year old car, exhaust rebuilt at one point in its long life, etc) but one stood out to me. When I am reving my way through 3rd gear I get a pulsing vibration. The frequency is a bit less than 1 second per pulse. Most noticeable at 2-4k rpm. Any thoughts on what that could be? It isn't super loud, but you can feel it in the seat. Not something I've encountered before and am a bit perplexed. It only happens in 3rd.

Happens right around 3000 rpm, right? Probably driveshaft related. Had an '81 242DL that did that. Replaced the driveshaft and output flange on the M46 with stuff from a Canadian GLT (same as a 1983 or newer 240 or a turbo 240). Vibration went away.

John242Ti said:

Happens right around 3000 rpm, right? Probably driveshaft related. Had an '81 242DL that did that. Replaced the driveshaft and output flange on the M46 with stuff from a Canadian GLT (same as a 1983 or newer 240 or a turbo 240). Vibration went away.

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Yep about 3000 rpm, my guess is maybe U joints? Center carrier bearing? My M41 used to do something that felt similar but in overdrive. New u joints and making sure it was in phase fixed that.

Probably. Also, cars with the butterfly trailing arm bushings frequently have longer arms, which may cause the driveshaft to be at a slight angle. So, that could be a factor as well.