Thanks Nancy, @Rae Reich and @Pres ! It does appear to be a bat issue...although the sound of the motor isn't "consistent" (?). I assumed the new masonite bat(s) I bought were nearly perfect but nope. I've been closing my eyes during centering and that seems to help (so I don't get caught in the rhythm of the wobble - ha!). Regardless, I'm ordering new plastic bats this week. I also used some shelf liner per @liambesaw 's recommendation and that has helped a lot!