Defroster Button + Squeeling = ??
 

Hi everybody. Haven't been around these parts lately as the ol slushbox has been runnin solid. Hope everyone is well.

Gotta little problem I've been lax on following up on, hopefully I can get some pointers:

A/C + Heat work like a charm, no problems there, but when I press the button for the front windshield defroster I get a cyclic (on and off) squeeling from up front somewhere. In addition to the squeeling sound the car runs a bit rougher until the squeeling stops. Only when i press the front windshield vent button (right below the recirculation button to the left of the fan knob).

Any help, as always, is greatly appreciated.

Thanks!

Defrost engages the a/c compressor. If the belt is a little loose, it could be slipping on the compressor. Try checking the belt tension or you can run just the a/c to see if the symptoms can be produced that way.

Does it sound like it's coming from the engine bay or the fan motor area?

I agree with Jeff...has to be the a/c compressor (because the engine runs rough) but what is strange is that you said the a/c works ok by itself. I'm not sure why the defroster would change the way the compressor works.

Perhaps the combination of the additional electrical load (from high fan speed) and the A/C compressor is enough to make the multi-rib belt slip?...
That or the A/C compressor clutch is going, but since it works normally on it's own I suspect not.

Good point Zero...didn't think about the extra electrical load. You may want to check the tension on your a/c alt belt.

Might be a low battery. Sometimes the alternator howls, initially, when the demand for amps spikes.