air conditioner howl

It sounds like a air leak. with even a small leak it can whistle, check to make sure the air ducts to the defroster are tight and in place.

Thanks for your quick reply.
If it is a leak in the ductwork wouldn't it howl in the heat or vent mode as well as the A/C mode ?
This is a LOUD HOWL ( like a tin horn ), not a whistle.
As i said , it only occurs when the outside temps are above 80 degrees and when the engine is at idle or less than 1000 RPM's
I can understand that the plastic ductwork may expand at high temps creating a joint leak , but , i don't understand why the howl stops when the engine gets above 1000 RPM's.
Can i get to the defrost ductwork without removing the dash (ugh) to check out your suggestion re the fittings ?

check the vacuum motors that move the dampers back and fourth, and the dampers themselves. Sounds like something is rubing on the fan when it's in defrost or ac mode (the damper would be open for the ac in either mode) . When at higher RPM less vacuum = less pull on the damper from the vacuum motor.
You prolly have a leaky one, vacuum leak, or someone kicked the underside of the dash hard enough to bend something in there.

Sounds reasonable.
Where is the vacum motor and how do i get to it and what is the check procedure?
Thanks