Belt squeak when warm

ok, another small problem...
99 LTZ, with a 3.8
after I drive the car for about 45 minutes, i get a squeak/squeal from the belt.
It only appears at idle or when coasting. As soon as you hit the gas, it goes away.
It has a new belt, and idler pulley. Any thoughts??
Ive checked the belt for abnormal wear, but I dont see any. The belt is about 3 weeks old.
Thanks!

Hey man im having the same prob with my 3.8L engine, if i where you id check your water pump, thats whats causing the horrible squeak on mine, and a word of caution if you can help it stear clear of refurbished water pumps ive blown two in the past week.