[41ford1]

That's a symptom of worn motor bushings. If it only does that when warm, the bushings are worn just enough to let the armature drag on the fields. Do the simple things first. Check all the connections including the ground side. If the starter is bad loosen the hex bolts in the end plate. These go all the way through the starter. They also hold the starter together. There may be clip from the top bolt to the pan. Remove it also. Be careful when you pull it out. You could end up will a 3D puzzle.