rubber burning smell, 88 Celebrity

I noticed a rubber burning smell only when the heat is on and it only smells under the hood and not coming inside the vents in the car, anybody know why I smell that? The car is a 1988 Chevrolet Celebrity 2.8V6 w/AC. it doesn't get to a burning ( as in seeing any smoke ) situation and it was like this last winter and I didn't smell it all summer, Any ides would help, thanks in advance.......John

The A/C pump will run during the time you have defrosters going so I am wondering if there may be a problem there.
Might be belt slippage caused by a lose belt or a compressor going bad.
Easiest way to determine if this is the problem would be to unplug the electrical connection from the compressor
 

thanks for the reply I'll try that, but  when heat is on compressor is not engaed?, only when defroster on, not sure if I had the defroster on when I smelled it, any other ideas if its not compressor,? would belt dressing help? My car has a spring loaded tension terpintine belt system.