Buzzing noise in tranny

It is specifically the pressure regulator valve bouncing back and forth in it's bore, so if you are rebuilding it anyway don't sweat it, as it will very likely be gone after the rebuild anyway.

As mentioned above, a plugged filter or being low on fluid can also cause it.
Putting it in neutral before reverse will work is a side effect of worn parts letting the fluid run out of the oil circuits, and that should also be solved with the rebuild.
(Especially if you get the modded manual valve that allows fluid circulation in park.)