I have 1997 F150 4X4 with a 5 speed manual transmission, when I'm in 5th gear at highway speed the transfer case is making a funny noise. It only happens in 5th gear at highway speed but does not affect the drive ability of the truck. I'm sure it the transfer case because if I apply a little pressure to the transfer case shifter it change the noise a little bit? Anyone have any idea what it could be?

 

First place to start is check fluid level

 

I bought the truck back in Jan and the noise was there when got it, I changed all the fluids in Mar. including the transfer case, engine oil, both differentials, and the manual transmission. Made sure to use the factory recommended fluid and double checked the levels last week but no change to the noise. But the fluid in the differentials, transfer case, and transmission were a nasty gray/black color.