a/c compressor

Two possibilities. The compressor could have bad bearings bit it's more likely the clutch is slipping. With the engine off, turn the front plate of the assembly by hand. If you feel roughness, suspect the compressor. If it feels smooth, even though it might turn hard, suspect the clutch. The clutch can be replaced separately but very often, to get the pulling tool in there, you have to unbolt the compressor from the engine.