Grinding Noises at Low RPMs

Q: What’s that buzzing/grinding noise at low RPMs?

A: It could be related to this technical service bulletin:

Bulletin #: 277142
Publish Date: 10/92
Models: 1984-1993 Firebirds w/ Hydra-matric 4L60/700R4 Trannys
Condition: buzzing noise from tranny at idle (more so when in reverse)
Cause: pressure regulator valve oscillating due to oil pressure instability at lower idle RPM
Correction: a new pressure reg. valve has been made to improve oil pressure stability at lower RPM (part #8684048)