I have an 03 Avalanche with auto climate control and dual zone HVAC. Recently the blower motor stopped working. I noticed a thumping noise coming from the blower cover under the glove box. Putting my hand on it, I could feel the thumping also. Oddly, if I turn the engine off, and turn the key back to on leaving the engine off, the blower works. When I restart the Av the blower stops and begins thumping. Turn the engine off and key back to run, and the blower works. I'm confused. At first I thought it was the resistor, but then when I turned off the engine and it worked with the key on I thought that wasn't the issue.

 

You likely have a bad actuator. The noise you hear is not the fan, but typical of a bad actuator.A bad blower won't make that kind of thumping sound.

 

Is the actuator located near the blower motor? I can feel the thumping on the blower cover also.

The thing is, the blower will work with the engine off and the ignition key in the "On" position. Would this still happen with a bad actuator?

 

Thinking the thumping might be the actuator for the blend door, I turned the key on with the engine off so the blower would come on. Then I used the manual switch to direct air through the floor vents, then dash vent, and finally the defrost. It all worked so I'm thinking it's not the actuator. Also, the thumping noise increased in speed as I increase the fan speed.

 

I could be mistaken, as I am not perfect. But I've never hear a fan "thump". They might make a whizzing noise, or similar. However, If your actuator is working fine then you might need a new fan, of course.