Heater only works on high... WTF

I got into my car today, turned it on, and put the heater on. But when I put it in the middle like I always do, it wasn't on. It doesn't turn on at any setting but the last notch, which is the highest setting. And when I turn it off, I hear this odd whistling sound for a few seconds. Whats the problem here, obviously some thing went bad, but what? Any help would be awesome.

Sounds to me like a resistor. Replace that and I'm almost positive that will fix your problem.

:1: Resistor pack took a shit. Fix it and you'll have heat again.

Thanks a lot guys, thats what my dad said too, but I figured I'd ask for a second opinion. Hopefully thats what it is, its the only thing that sounds right.