Knock/rattle at idle in my '93 PA

Two weeks ago we had some heavy rain and afterwards at idle the engine would squeak, today that evolved to a decently loud knock/rattle that disappears above 1500 or so. Not running rough and no check engine light. I'm guessing some water got into one of the belt pulley bearings and toasted it, any good way to diagnose this thing? I've tried the whole screwdriver as a stethoscope thing and can't pinpoint a spot. Ideas? TIA,

-bugdude

Check the harmonic balancer outer ring to see if you can turn it by hand. (Be sure the engine is off)!

Attached to what appears to be the A/C compressor? Yep, turns by hand; I hope that's a good thing. Just turned the A/C on and noticed the rattle got significantly louder. Crikey, I miss my '66, that thing has no harmony and no balance...

Hmmm, it appears I was looking at the A/C clutch. Is it generally just the bearing that goes bad and is that something a shade-tree mechanic like myself can tackle? I'm not quite as inept as my lack of harmonic balancer knowledge might lead you to believe.

Here is a pic of the back side of the harmonic balancer. If you have an Ultra, there will be two pulley groves for the belts. The "harmonic balancer" is the main drive pulley mounted on the crankshaft.

http://img115.imageshack.us/img115/7060/1996balancermy6.gif