Could it be the aux drive belt tensioner? With the a/c engaged if it were the compressor bearings this would stop when you switch off the a/c. If it were the mag clutch bearings this would stop when you turn on the a/c.
Whip off the aux belt and spin the various pulleys by hand and check for any dryness / roughness..
No harm done in running the a/c all the time- it will prolong intervals between needing it re-gassed...
Tim..