I posted an earlier problem with my 1941 9N that a growling noise was coming from the trans. With depressing the clutch, there is no noise, and there is no noise with the tractor being stationary, and idling in neutral either. It growls when I am moving, in gear without depressing the clutch.

Can it be the rear end? All oil is ok.

Some gear noise is normal.It could be gear noise or a wheel bearing.I get some noise on my 9N. It has been doing it for decades.Unless the growl noise is excessive, I would take no action.