Heater smokes

[atcavage]

On cold mornings (go figure), I'll turn on my heater and defroster, and after about five minutes (once it heats up), I get smoke. If I turn off the heater, smoke will gently creep out for a while. If I crank it, smoke will pour out and fill the cab until it runs out of whatever is causing it. I haven't been able to reproduce it except driving on a cold morning, and can't figure out what the problem is.

I've heard leaves rustling around in there, but I'm pretty sure I got them all out. It might be electrical, but I've experienced no electrical problems and the smoke doesn't smell like an electrical fire.

I've had no other problems, and the fan runs fine. Not a major issue, but I want to run my heater without walking into work smelling like I was just at an early morning barbeque.