Check the hole size in the fiber brush plate. It should be about 0.126" when new. The armature shaft is 0.125" (1/8").
My guess is that the hole worn too large and the armature is contacting the field plates. The good news is the the opposite bearing in the whistle box is a real one and is probably fine. No need to remove the armature which requires popping out the whistle box backside rivets.
You can repair by purchasing a new brush plate (~$6) Or by drilling out the fiber brush plate armature hole and fitting a real bearing. There is a bearing, part number OM-3, that works nicely for this BUT they don't seem to be sold by any parts dealers that I could find. They are used on at least 1654, 1655, and 1656 locomotives as the armature bearing opposite of the brush plate.