Road Hog

Also check where the rod from the handle goes to the valve--sometimes the rod at the valve has the retaining pin-then the rod and knob will unscrew from the valve

It sounds as if the interaxle differential may be making the noise-it can be caused by numerous things-make sure all tire are the same size and properly inflated-if everything at the wheels is good, the interaxle buttons or cams oould be worn which csn cause the driveshaft to slip almost a gear slipping-if that is the problem the noise will almost be a ringing sound (resonating through the driveshaft-it also could be a broken spider gear in one of the rears but that usually only will make a noise when cornering or if one of the wheels are spining-the easier check is the interaxle differential which if it is a Mack rear, is right in the front section of the front rear-