Air conditioner makes screeching noise?

If it only makes the noise when the AC is turned on, (which includes the defrost mode), suspect a locked-up compressor. Try to turn the front plate by hand. If it is free, look also at the cooling fan. You can unplug the condenser fan to see if the noise stops, but a squealing fan will usually also be heard when the wind spins it when driving at highway speed.

If neither of these are obvious, unplug the compressor clutch. If the noise stops, look closer at the compressor.

Caradiodoc