92 burb defroster

I got a 92 suburban and the defroster don't work correctly, the motor that moves the deflector is working but no air comes out the defroster vent, I
get a high pitched whistle so I think maybe something has fallen down in there. Has anyone tore into the duct work under the dash or had this problem before? I guess my question is, how and where do I start to take this apart and find the problem?
Thanks, Tim
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