1996 Ford Mustang Burnt nasty smell when I turn the heater

How can I find out what is causing a smell when my heater is on? Not a gas smell, and I don't know what coolant smells like, and it's not sulfurous. First time I smelled it I thought it was tires or brakes because the odor reminded me of burnt rubber. But then I had just started the car, and turned on the defroster with high fan, and the odor really showed up. No smell if I turn the heat off, no smell using A/C. No wet spots under the dash, no visible sign of dripping liquids under the car. Oil pressure is fine, temperature is normal, coolant level is full. Only recent repair was 6 months ago when Ford dealer replaced the intake manifold, and there was no odor until now. I've talked to 2 different mechanics, one says it might be a leak, another said maybe the head gasket (which I doubt because the performance is still great and there is no visible residue around it.) Brakes were checked and are fine. Just a nasty smell when the heat is on.