Sounds like belts squeeking.

[00Bravada]

I have a 2000 bravada and it sounds like the belts are squeeking. On cooler mornings the noise is loud and as the engine warms up it goes away, but the last few days notice getting worse. I put some of that belt conditioner on the serpentine belt and noise didn't go away. Noise only started to go away after about 1/2 hour of running the engine. I thought it might have been the belt but doesn't seem like it. Any pointers out there of what this might be? Fan clutch belt, tensioner, or do I need new belt?

[Chris Stewart]

If it has many miles, I'd go with a new one. While you have the old one off, spin the pulleys and check for smoothness.

[00Bravada]

I replaced the automatic belt tensioner assemble and the noise went away.