GM issued a TSB for that issue for the 2000 and 2001 model years, that should have been already applied to yours.
Date: September, 2001
Rear Brake Moan/Hum Noise During Brake Apply
(Replace Rear Disc Brake Pads)
2000-2001 Buick LeSabre, Park Avenue
1998-2000 Cadillac Seville
2000 Cadillac DeVille
2001 Oldsmobile Aurora
2000-2001 Pontiac Bonneville
This repair DOES NOT apply to rear brake grunt/groan, pulsation or squeal.
This bulletin is being revised to add a model and to update the model years. Please discard Corporate Bulletin Number 00-05-23-001 (Section 05 - Brakes).
Some customers may comment on a moan type noise coming from the rear of the vehicle when applying the brakes. This condition may be more prevalent during city stop-and-go or mountainous driving situations. In rare cases, this moan type noise can also occur during the first or second application of the brakes when the brakes are cold.
As the vehicle slows down from 48 km/h to 0 km/h (30 mph to 0 mph), the rear moan type noise may produce a low resonating "hum".
This condition may be due to pad vibration being transmitted to the caliper assembly resulting in a low frequency resonance.
Replace the existing rear disc brake pads with a new rear disc brake pad service kit.
The new rear disc brake pads will reduce the vibration being transmitted to the caliper assembly.
There are several good brands of pads that could lessen the problem. They may have shims attached to the back, or have a special coating. Some will bevel the edges of the pads, and others may have a slit down the center to lessen the vibrations. If you want to try some new pads, go to a good auto parts store that has a knowledgeable person at the counter, or you could try some things on your own, with your current pads.
Buy some disc brake quiet, and apply it to the back of the pads, after cleaning them well. Also clean up the caliper piston. Clean up the metal pad clips well, and apply a touch of brake lube to them, as well as lubing the caliper bolts and pins well. You could also try filing down the side edges of the pads a touch, with a file.
A '92 has drum brakes in the rear, so is a totally different animal. Your best bet is to pull the drums, and inspect the brakes. Check the brake wheel cylinder for leakage, and look for any broken springs. If the cylinder is leaking, replace it, clean up the drum well, and replace the shoes. Also, make sure the brakes are adjusted properly. If you need additional help, that would be better posted in the 92-99 section.