Frictional Force in a DC Motor
DC brush motors make electrical contact with the rotating armature with spring-loaded carbon brushes. The brush contact is a major source of friction. The armature bearings (at each end of the motor shell) also generate friction, but far less than brush friction. In the photo below, the red and blue brushes are pushed against the commutator:

The armature frame containing wire windings is quite heavy. While rotating, it acts as a flywheel, smoothing the rotating motion. Any offsetting weight on the motor shaft will likely cause vibration, and bearing friction will increase, since rotating speed of these motors is quite high. A high speed also keeps rotational rate nearly constant, although the extra friction will likely cause rotational speed to be somewhat slower, and current drain somewhat higher.