It's the A/C clutch (pulley) bearing. Nothing's wrong with the compressor.
I used tomcollin's instruction in replacing mine, skipping steps 4 & 5.
http://www.automotiveforums.com/vbul...light=harmonic
Ops, your condition was opposite of what i got before i replace my A/C clutch assembly. So, disregard my answer above.
I got a noise when A/C was OFF (compressor was disengage from pulley) and i didn't have a noise when ON (engaged).