[GRutter]

I went back and read your Dec 2019 thread regarding this problem. At that time, you described this as a "screeching noise" when decelerating in 1st & 2nd gears.

In this thread, you are describing the noise as a "metal banging noise".
Have the symptoms of the problem changed with the work you have done ? Metal banging sounds more serious than screeching.

With a rebuilt engine, I'm going to assume that the thrust clearances are correct, and therefore going to rule out the engine as source of noise due to deceleration.

I would agree with the logic of rotorwrench... that the problem is in the transmission... as the noise only occurs in 1st and 2nd gears. As he stated, in 3rd gear the shafts are directly coupled and you stated that the noise does not occur in 3rd gear.

I would want to see if you could also recreate the problem in reverse.... although it may be more difficult to duplicate the forces from accelerating / decelerating / engine braking while driving backwards.

You indicated that you rebuilt the transmission, but didn't give any detail as to what you found or replaced. Let us know what you kept vs what you replaced when rebuilding the transmission.

Is it possible to upload an audio file of the sound it is making? Might help to diagnose the problem.