Perhaps this TSB will apply? I don't know, as I'm not sure that I understand their difference between "moan/hum" versus "grunt/groan"
Rear Brakes - Moaning/Humming Noise
File In Section: 05 - Brakes
Bulletin No.: 00-05-23-001A
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. Refer to the Disc Brakes sub-section of Brakes in the Service Manual for replacement procedures.
Part number for the Bonneville'* is 18043999
Perhaps your new brake combination isn't playing well together?