Blend Door (I think?)

I think I have a blend door problem. When I turn the key on in my 1991 GMC, there is a clicking noise for about 20 seconds then it stops. I hear the same noise if I hit the recirc button on the climate control. I think it could be a blend door trying to open. The heat/ac works fine, maybe it is the recirc door. Does anyone know where I can get instructions on checking/changing this?
Thanks,
Terry