Buzz from HVAC - heat no longer works but AC does

Advice and ideas please.?
I start my 2004 4Runner. I immediately hear a buzz. I turn the HVAC fan on full and the buzz goes away. But now the heat does not get hot. The AC is still cold.
Ideas?
What could the problem be?
How serious is the repair?

 

The "buzz" if coming from the glove box could be something like a leaf stuck in the blower motor. One way to check is to look at the filter.

The heater and A/C issue is probably the blend door actuator. You can replace the actuator pretty easy.