Need some help on high pitch whistling noise when fan is on

[UATahoe]

Ok. So i just recently finished putting new weatherstripping everywhere in my 87 pickup. Now when i turn on the fan i get a realy high pitch whistling noise. Here is the strange part though. If i put it on max AC, it is as quiet as can be on every fan speed and not a single problem. Hear nothing. But on any of the other remaining settings (reg ac, vent, heat, defrost) there is a really loud whistling noise on all fan settings except the lowest setting. And you can almost feel the pressure effecting your head. I am stumped on this one. Any help????