Windshield defroster is whistling ...

[shibbershabber]

92 K1500 Sub

When I turn on the windshield defroster at low speed it doesn't seem to have much effect.
When I turn it up to the higher speeds it whistles through the vents and still isn't very efficient

As if there is a restriction in the vents...

How can I check, is there something that I am missing? its only when I use the windshield defroster.

 

[bdubyah]

Maybe the actuator that moves the airflow to the defrost vents is sticking? I know on my 93 when it's cold and I switch to defrost, it doesn't move all the way and it kinda whistles and I get almost no air coming through the defrost vents. Usually have to cycle from the vents to defrost a few times to get it to click all the way over.

 

[dirtridinz71]

Agreed that the blend door is sticking. The actuator for it is just above and to the side of the gas pedal.

 

[94Sierra4x4]

Mine does the same thing on all settings. Whistles like a sumbitch on high, unfortunately actuator cable is gone, plastic steering column cover that it hooks into was destroyed, somehow came off other end too.