If its on the 1533 MF I wouldn't be supprised if your front axle is about to go... iv seen a few issues with those and my 1540 is now making a nice screeching noise while in 4wd.. most likely I got play in the driveshaft and will need to break in down soon... I only use the 4wd when absolutely necessary... I also hit the lugs about every 10 hours
Check the gear oil in the front end. My 4110 started doing that in 4wd and I topped off the gear oil and it stopped.