It should have a slight down angle when the car is flat to the ground. If the pinion angle is off it can cause a problem but I have not heard one hum because of that.
You may try an old trick and drop in a can of STP to see what happens. It will thicken the
oil and if it is in the trans help knock down the sound. Of course this will not fix the problem but it will help you locate it.
Are you sure it is not rear gear hum you are hearing? Did you have the rear end redone with a new set of gears and bearings?