can you describe the noise? or, post a recording with audio?
I also find those $5 mechanic's stethoscopes SUPER useful for isolating noises. In particular I've used them to identify wheel bearings and idler pulley bearings, they make a grinding noise when you use the stethoscope.
That said, I'm not sure a bearing would explain what you've got here.
odd that the noise goes away when it's engaged. I've never heard of this, but just thinking logically, what if the clutch is somehow scraping when the a/c is off and then when it locks up when you turn the a/c on, the noise would go away? (I'm just thinking through and making up a theory). Other than the clutch, I can't think of any other part that could STOP making noise when the a/c is engaged. wouldn't be bearings. wouldn't be the belt.
also just noticed your greeting, 'aloha.' would you happen to live on Oahu?
I'm here in Aiea
with a yr 2000 toyota sienna xle, 121,000 on the odometer.
my own a/c has been behaving strangely, but it turns out I can replace pretty much the entire a/c system for $500 if I do the work myself. compressor, condenser, drier, relay, triswitch, and both hoses, and $100 for an evac/recharge. NOT doing the evaporators (there are two, front and rear), without definite cause those are just so much work to get to.
so rather than chasing waterfalls trying to diagnose it, I'm just replacing everything. a terrible habit (parts swapping) but i figure with everything being 20 years old and original, I'll come out ahead by just doing it all.
I did the same thing last year when I opted to just do all the cooling hoses and radiator, when one hose started leaking. I don't trust 20 yr old rubber haha.