a further and final note:
I've resolved the vacuum locking problem for the door locks on this 560SL, details in different thread. Turns out that thread and this one were inter-related.
The buzzing noise is most evident when one unlocks the doors, not when one locks the doors. At least when locking it appears much more muted.
The locks work so for now am going to adjust to living with the buzzing noise when getting into the car, at least it times out pretty quickly.
No other vacuum related problems that I've noticed
Thanks very much to all who responded and helped.
Network Integration Engineer
MCSE, CLP, CCNA