noise

I purchased a 2006 Nissan Frontier in Nov-05. One day last month after I washed it .I started it up and it started making squeeking noises like the belt was loose or like the pulleys needed lubricating. I noticed after it warmed -up to normal temperature the noise went away. A couple days ago I backed truck out of garage and let truck idle til normal temperature .I shut off engine after normal temperature. Truck set for about a good 30 mins to 45mins.Then I started it up .It started making the sqeeking noise ,I opened up the hood to try to pinpoint. it sound like one of the pulleys were sqeeking, but of course it could have been something else. I drove it to the dealership of course by the time I arrived the noise had stopped .I left it there overnight then they started it that morning. they didnt here any noise, This is an intermittent type problem and it is frustrating . What do you think this could be? Or do you have any suggestions ?