Engineer, I had absolutely, exactly the same symtoms on my '91 SE last summer. It was indeed the reverse (b3) clutch plates worn out. When they got thin enough, they allowed the piston to move too far in, and it began striking the K1 housing fingers. Click, click, sounded like an old speedometer cable clicking when moving in Reverse. After the tranny heated up, it expanded just enough to stop the clicking.
I bet it's kinda slow to engage reverse when shifting from Park, too?
Upon opening the tranny, I found little pieces of B3 clutch plates in the pan and filter, and I found the little rub marks on the K1 basket fingers.
This is metal-metal contact, so I would recommend you not let it go.
The b3 plates were shot, and just starting to delaminate. On the other hand, the K1 plates looked absolutely new.
I went in just as far as I dared, popped in new b3 plates, and buttoned it up. So far so good.
Do a search under some of my posts last spring (S-class Guru) for more comments.

DG