When I have my ac on or heat on it makes this squealing noise. When I turn it off no noise. Could it be my belt I know there is two. The second belt I thinks runs the ac compressor. Is there a pulley for the ac compressor.

I took it to my mechanic and he said it'* the bearings in the ac compressor unit and that the whole unit is needed. But I had an issue with my temp staying in the cold no matter how long I had the truck running. So it happen to be my coolant temp sensor change that and now the temp is where it should be. I notice when I turned my air on, no more squealing Noise or anything. I don't no its weird see how long that stays that way anything I change on this truck is cause it'* very old and with 176k anything can go wrong