My experience (w/ other vehicles w/ same symptom) has been that the a/c "clutch" doesn't release all the way when not in use. Clearance between the driving element & the driven element is very small & any variation in spring preload tension doesn't allow the driven element to release completely.
The "spring" preload appears to get "tired" after a while (I can relate). Other than the noise (which typically goes away after a while), the problem is not serious unless the clearance is such that the compressor keeps rotating sporadically; if so, replace clutch unit.....my $.02.....
