I have lived with this problem for years and have always wondered why.
I had a factory rebuilt motor and a new clutch installed by a local shop
and it chattered immediately. It occured in reverse under a light load
such as backing uphill, or in 1st under heavy load only. The shop replaced
the clutch disk & pressure plate with new parts twice, but the problem
persisted. I had so many problems with the first shop that I finally gave
up in disgust. Later on, a different shop replaced the slave cylinder, but
there was no improvement. I got fustrated with the whole thing and have
learned to live with it. If I baby the clutch I can avoid the chattering
about 80% of the time.

Walter
85 Vanagoon 1.9