i got an 04 f 150 fx4. went mudding and then it starting hissing/squeeling on me. not a rock in brakes because whenever i start engine the sound starts. when i give it more gas the squeeling increases with the rpm's.

 

its like a bird chirping cuz its not as loud as a ****ty car would sound if it were squeeling.

 

Sounds like the belt or one of the pulleys. If its under warranty, then take it to the stealership.