Just swapped an m41 into my 122 wagon along with a short driveshaft from the parts pile, and trying to pinpoint a new noise that has appeared. I went through the tranny and cleaned/inspected everything and replaced all of the seals, the inside was very clean when I opened it up and everything looked to be in great shape. I checked the u joints on the driveshaft for slop, and the centre bearing seemed to spin freely on the shaft. In general, the new transmission shifts very well and the OD works beautifully, but there is a funny sound that wasn't there before. The sound is a rhythmic sort of scraping sound coming from the driveline, and only appears when coasting at 40mph plus or so, and only sometimes. It doesn't matter whether the car is in gear (any gear) with the clutch in, or in neutral with the clutch in or out, the noise remains unchanged. In other words, it appears when the driveline goes slack. As soon as I put power to the rear wheels or downshift to put some tension on things in the other direction it goes away instantly. It comes and goes depending on the day but usually once it starts it keeps going. Could this be the driveshaft bearing (I hope!) or could it be something in the transmission itself? Maybe the rear end?

One last thing, I did replace the transmission mount at the same time, and it seems like it's holding the tranny a bit higher than the old one as it's not compressed yet, could that be putting something out of alignment? Ideas?