The m46 in my '84 245 is getting noisy. I am trying to isolate the problem to the transmission or the clutch and I think that I am leaning toward the transmission.

This M46 is original and has over 400,000 miles on it. Symptoms are gear or bearing noise in 1st, 2nd and 3rd. The noise is most noticeable when coasting to a stop with the clutch pushed in but I can hear the noise while accelerating through the gears, too. The pitch of the noise is related to the gear that I am in, which is why I believe that the problem is internal to the transmission. For example, if I push in the clutch at 30 mph in 3rd gear and allow the car to slowly coast to a stop, the pitch of the "hum" drops as the car slows down, but then when I downshift into 2nd gear (without letting the clutch out) it picks back up again.

I also get a nasty "growling" sound when I let off the throttle in 3rd gear.

Center carrier and u-joins are a few months old. Clutch is about 3 years old.

Any ideas? Try synthetic fluid and see if that buys more time?