Went back and reexamined everything as before. found nothing out of place. Decided to look again at the drive belt which looked good on the outside and inside. Come to find out that the belt has a 1/2-inch spot missing on the inside. I am assuming that when this passes over the drive pully that it is causing the thumping. waiting on new belt to see if that is it.You’ll probably going to have to check under the unit, I’m thinking bad pulley on the drive tensioner arm. Post unit model and serial numbers from under the seat.