Belt making noise when the defrost is on

Sounds like problem gone when AC clutch disengaged when switched off. Could be AC pump belt burning from seized or the belt slipped drastically. Is the smell like burning rubber or oil?
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Make model and year of vehicle.

My guess would be you either have an oil leak that is causing the smell and causing the serpentine belt to slip, or you have a pulley for the serpentine belt that is not spinning freely and causing the belt to slip. A slipping belt can also cause the burning smell that you describe. You might have a AC compressor clutch that is failing and not freewheeling correctly, you could have an alternator with a bad bearing, you could have an idler pulley or tensioner pulley with a bad bearing, etc. I'd certainly be checking for an oil leak as well though.

you more than likely have a sciezed air con compressor. most modern cars have climate control which uses air con even when demisting and when engaged the electric clutch tries to engage but the pump itself will not turn, clutch will get red hot and burn your belts

You should take the belt off and examine it. If that is what was starting to burn, you will have to replace it. And now check all the pulleys to see if one of them has siezed up. Spin every one around by hand, feel for roughness or stiffness, or if it wobbles. If you find one that is siezed, replace it. That is where all the squalling started.

when defrost is selected a/c clutch is engaged and system automatically draws fresh air instead of cabin air.i woud strongly suspect compressor belt too loose and slipping or compressor seized and slipping belt.very easy to confirm visually.ever any squealing noise associated with odor?

You most likely either have a loose belt and it smells like burning rubber or you have a small animal caught in the belts.
Replace or tighten the belts and you will be happy

Thanks
Gary

a/c compressor may be seized. A/c compressor kicks on when in defrost, a/c, floor/defrost modes any time temperature is above 40 degrees F. That will burn the sepenine belt and stall the engine.
Try turing heat on floor mode only & see if ok.

Sounds like the AC compressor is going bad. Have the compressor tested and I'm sure the belt is what you were smelling. make sure nothing is touching the AC belt. If the compressor stops working properly the belt will burn. Solution= replace compressor and belt.

Please have your exaust pipes checked, perhaps a mount is burning. The catalytic converter can make a nasty odor when it is clogged. Check the wear on the serpintine belt.