From a 1996 Chrysler bulletin (
http://www.dodgeram.info/tsb/1996/05-09-96.htm) :
Groan Noise On automatic transmission equipped vehicles, a slight groan may be heard when coming to an abrupt stop or when allowing the vehicle to creep forward slowly with the brakes slightly applied.
What you are probably experiencing is called stick-slip. It is a low frequency vibration cause by the slip between the rotor and the friction material; more pronounced with a wet friction material (pads) when the brakes are lightly applied and the vehicle is rolling slowly.
I notice it when I have the radio turned down and the brakes are wet, usually from driving through some deep puddles. It tends to go away when the pads burn off the moisture; however, if I listen very carefully it will still be there, but not as pronounced.
This is probably what you have, and there is really no fix for it. It does not affect the stopping of the brakes (unless the pads are saturated).
But, not a bad idea to let the dealer check it out to make sure the pads are ok. They are bonded to the pad plate, and one may have lost adherence to the plate, very unlikely, but still possible.