Riding Lawn Mowers

If this is happening with the deck disengage I would suspect on the drive belt idlers have gone bad. Most times when I seen a bad idler the flat spotted it is the flat idler from the bearing locking up. You need to remove the deck and check the drive belt and idlers. There is two v-belt and one flat idlers.

If the idlers are noisy when turn by hand or badly worn then they need replacing. Personally when I usually end up replacing all three idlers and belt drive when I servicing that area. The v-belt idlers are made of plastic composite that wear fairly heavily causing drive belt slippage.