if it's grinding, take it apart, they are simple, remove the screws from the bottom and slide armature out. be careful not to loose the brush springs.
check the brushes for excess wear and blow it out. it's probably excess graphite from the brushes.
most electric motor shops will have brushes for these.
put a little di-electric grease on the ends of the armature. spread the brushes over the armature when slidding back together.