Curious Thread Title
From what you state in your OP, why do you suspect the AC at all?
Noise change with speed?
Do you run a cone filter? Sure it isn't hissing coming from there?
I never heard of using WD40 around belts or clutches.
Could be weak tensioner.
It COULD be a bearing. There are several accessories with a bearing running off the belt.
You first noticed something when running wet. You might spray water on the front and see if it repeats the problem.
If convinced it's the AC, then wiggle the clutch disc to see if heat-warped against the plate. Bearings don't whistle. They usually growl, chirp or squeak.
\'96SS SOB: SSRI, Herter Tune, Tri-Y II, , 3000 Edge, F/HO bars, METCO extendeds with CV MMC, Bilsteins, currently Vredestein Ultrac Sessanta (315/35 rear). Finally, wait for it... LT-4 knock module!
\'96FWB BUW-Butt Ugly White: PCMPerformance tune, HDFA w/ swissed filter, 1/2 SLP s.s. catback with H- and Flow Super 50s, oem opti rebuilt 3 times, SS Bars, METCO LCA, Airlift 1000, Monroe SS, PowerTrax No-Slip, CTS wheels with +1 rubber. Finally, yes again... LT-4 knock module!