Nissan Repair: A/C, margie

Question
My car makes a shrieking noise that comes and goes when I turn the ac on...what could it be??  It has made it when the heater is on as well but not not very often.

Answer
Margie,

The noise is probably a loose or worn belt.  I would change the belt.  When the heater is on there is a defrost mode that runs the air to dry out the heated air.  I say it is the belt since you did not say the airconditioner quit working.  So, just change the belt.