If you have scrape marks on the outside of the armature, and rub marks on the frame, then the bearings are worn out allowing contact. This effectively shorts out the motor, which causes way excessive current draw from the battery. That's why the cables get hot. The current draw makes the voltage fall way low, which disengages the solenoid, which then makes the voltage com back up, and the cycling of the solenoid makes it buzz. This is the same sound a weak battery makes, even when the starter motor is good. The battery just can't keep the voltage up when more is demanded than it has in capability, old or new.
Had this problem on my 75 CB550F about 15-20 years ago. I robbed a motor bearing from another Japanese starter I found in the wrecking yard, and put it into the failed one. The starter motor still works fine to this day.