Alternator is being hit from behind. How do I stop it?

check for loose motor mounts~ should have plenty of "wiggle-room"

make sure there are no broken bolts~

Could be the nut behind the wheel

Only a gremlin can do this, a person cannot fit under the hood and whack and alternator while driving down the highway. If you do catch him do not feed him after midnight or bad things will happen. Then locate your local shaman to take the creature back home.