I've got a 2001 Sierra that's not blowing any heat when I put the climate control setting to heat and the fan on. If I leave the fan off, I ...

I've got a 2001 Sierra that's not blowing any heat when I put the climate control setting to heat and the fan on. If I leave the fan off, I feel some heat (not a lot) when I put my hand up against the vent, but when I turn the fan on (even on 1) the air feels cool. I've turned the a/c button off, same results with the heat. I did notice that the a/c compressor cycles on and off, even with the a/c button off, I'm not sure if that's supposed to happen. So, I disconnected the compressor by unplugging it from the dryer hoping I'll get more heat and nothing changed.

Any ideas on what's wrong here?

First things first. I think you may have a plugged heater core. When the truck is warmed up, feel the in and out hoses to the heater core. If one is hot and the other cold, I bet it is plugged. Have you changed your antifreeze recently? Added any bars leak to the coolant to stop another leak someplace?