GM / General Motors: a/c compressor "thrum" - 1997 Trans Sport

Beginning this fall, I've noticed a loud "thrumming" or droning sound coming
from under the hood of our '97 Trans Sport that didn't used to be there -
only with the a/c compressor engaged, and proportional to engine speed.
Sometimes I even hear the same sound emanating from the rear a/c evaporator
(interesting front-to-back stereo effect!)

Although the a/c still seems to be working, this sound suggests that
something's not happy - compressor bearing? low refrigerant? Should I get
this attended to asap, or can I wait til next summer?

Any advice anyone has to offer would be appreciated!

Could be overcharged, and liquid freon is entering compressor. Does it do
it at idle ? If so, watch compressor while engaging. Does clutch turn
evenly, or does it make and break. Some clutches cycle, but the engagement
should be smooth and complete.