i have noitced reciently that when the transmission is under load, it squeeks. dont laugh but the squeek sounds like a bird? is it bad trans mounts, something with the clutch, or a bird stuck between the tranny and body, or somthing else?

my drive way slopes down, when i come to the bottom and use the engine to slow the truck (letting off the gas, but not diengaging the clutch) the truck squeeks like a son of a b***h. im hoping that this is nothing major. if it is the mounts how hard are they to change?
Zach