Hello, 1993 Blazer 1500, when switching from floor or mid heat to windshield defrost there is a pretty loud knocking for about 15 seconds under the dash right and then I can hear the blower trying to push air up but its restricted to a trickle of air getting up on the windshield.


thank you

 

sounds like a electric blend door going out. they are made of plastic and the gears strip out over time. i have the same set up on my 1990 c1500 but thankfully they have not stripped out yet. there should be 2 under there they are like little black squares bolted to the side of the hvac box with a few wires going to them . should be roomy enough under there to pop you head under there and find out which one is knocking and replace it. pretty common issues with impala's too