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!
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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.
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