Here is a trick I sometimes use to pinpoint noise.
I tried your suggestion I think transmission. So I took it out. I will add photos to make my gun finds clearer.
1. I could push the brake cog in direction of my finger, brake release drum turns.
2. I could turn shaft 6 or 8 turns clockwise looking down and it jammed that no more turning
3. I can turn shaft counter clockwise looking down and it turns, sometimes like gears rubbing
4. The shaft can be wobble side to side and moved up and down about 3/16 inch
5. Motor and coupler turns they are still connected together still, no transmission shaft movement
6. Clutch, I could not push with my finger and I took out to wipe out dust.
Does this suggest it is transmission has failed?