2005 Nissan Sentra squealing noise under hood

When I start my car in the morning, it starts squealing loudly. After it warms up, the noise will usually diminish. We have had this happen on and off since we have had the car. I have taken to the NIssan dealership several times and they would fix it or so I thought. It would last a while and then go back to squealing again. I have had all the belts checked and was told they are fine. We cannot seem to figure out what this is and evidently the Nissan people can't either. Now my warranty is up.
Can you give us some suggestions as to what it might be.