What year and model?
By third member do you mean the differential in the center of the axle?
Could the noise be coming from the driveshaft? These things turn all the time whether or not it is in 4WD.
Do you have a parts manual and owners manual? If not, you can order them online.
For a quick reference, some dealers have online ordering systems that will show you the exploded parts views. Affordable Tractors and Circle G tractors are a good place sto start.