240 possible bad transmission.

I have an 83 242 glt with a m46 transmission with 160k miles. When you shift to 5th gear there is a noticeable rumble feeling above 50mph / 2400 rpm when you are getting on the throttle. When you back off fully on the throttle there is a loud moaning sound. Initially I thought that it was exhaust sound but then you push the clutch in the noise goes away. Is this is that the sound of a bad transmission?

Sounds to me like a bad driveshaft U-joint. They're sensitive to load. I've had a few cars that only did it when you were going fast. Push in clutch and load goes away. You could just disconnect it (a few bolts) and try wiggling the joint to see if it's stiff. Really cheap to fix (less than $15).

I'll put it this way: If it doesn't have a leak, I have never heard of a Volvo M46 that failed from wear. They either fail catastrophically (from very high torque in a modified car) or they keep going forever. The other likelihood would be the electronic overdrive unit failing, but this is a failure mode I've never heard of. Really, check the driveshaft.

I had the same issue at approximately the same speed. At first I thought it was my exhaust mounting (I had just replaced it) but it was my u-joint. Went away immediately after replacing it.

I've taken the drive shaft out and had a drive shaft specialist replace the front, rear and center u-joints, and then rebalance the shaft.

Money well spent.