We have a 71CR in our barge. It gave many years of good service until an internal split washer broke and got munched in the gears. The oil circulated the steel bits throughout the box, but it kept going until we replaced the whole box with a rebuilt unit. At this point I learned a bit about Velvet Drives. The front half (well a bit more than half) contains the oil pump, planetary gears, clutch pack etc that give forward and reverse drive. The rear part contains just a shaft if 1:1 ratio (not likely in a barge), or another set of planetary gears to give the required step down ratio. Ours is 2.91:1. We bought our rebuilt box in the US for $1,650 USD (no core required) and paid $440 USD to have it air freighted to Schiphol.
Since the drive plate is attached to the engine flywheel it always rotates in the same direction, regardless of the box being in forward or reverse drive, therefore I doubt your problem is the drive plate.
One thing that does change in forward/reverse is the push/pull on the prop shaft. If you don't have an external thrust bearing in the prop shaft (we don't) the axial thrust in the propshaft is taken by two thrust bearings in the rear part of the gearbox. If the one taking the forward thrust is breaking up while the one that takes the reverse thrust is still good, would this not give your noise symptoms? I believe that if you have the room you could get at these thrust bearings by unbolting the rear half of the box and sliding it back while leaving the front half of the box still bolted to the engine. I didn't have the room to do this in our case so had to take out the entire box (about 70 kg).
Good luck, and keep us posted.
Trevor G.
Barge 'Penelope's Ark'