Burning rubber smell in recirc only

[turbodaytona87]

I recently replaced the heater core in my car and it seems to work alright. The heater works better when on recirc, although that brings up two issues. One, I thought because recirc was on the cold end of the selector, it was only for A/C and wouldn't work with heat. Second issue, sometimes I will get a burning rubber sound in recirc only. If it starts smelling like burning rubber, and I move it over to the heat setting then the smell goes away.

What would cause that, and it the temp door working fine?

[wallace]

Could you have gotten some debris on the resistors for the fan speeds?

[turbodaytona87]

But it doesn't smell when on the normal heat mode, just on the recirc mode