First thing I would do is drain the diff oil in a clean pan and check for metal pieces or shavings. The drainhole isn't hardly big enough to get much of a look inside, the only way to really inspect it is to remove the axle assembly from the tractor and remove the diff. from the housing. I would also drop the driveshaft and check the splines & couplers on each end of it - you have to remove the tube surrounding the driveshaft at the same time but it's not very difficult. Then again, you might have a final axle or drive problem and the noise is being transmitted thru-out the axle assembly. With the front wheels off the ground and the driveshaft out, you'll be able to pinpoint the source of the noise rotating the wheels and/or pinion shaft by hand. Have a friend turn the wheel and use a long 3/8" drive extension as a "stethescope", holding against different areas to loacate the problem area. I've done my share of axle & diff. work on my 4110, believe me, in can be anywhere in the front axle assembly. Oh yea, each final drive has a drainplug too, if the noise is in one of them you can drain the oil in the clean pan as for the diff. to check for shavings etc.