a/c condenser or compressor to be replaced

I just had my 1990 sedan 1.6 FE a/c retro fitted and recharged and now have cold cold air... woopee! . Now I have a siren like sound that is audible when the motor is started regardless if the a/c switch is on or off. A mech tells me it could be either of the subject and I don't know the difference between the two a condenser or a compressor. He seemed to think without seeing the car that it could be seals or rings in the unit (whatever one) that have dried out after 2 years of not being used. The a/c belt was removed 2 years ago. Can anybody suggest what's going on? How much a new compressor might run and if a used one is a good idea?

Many thanks