Husqvarna TS146XD Drive Belt or Transmission Slips

My one year old Husqvarna TS146XD just decided not to go uphill anymore - very reluctantly at best. I am assuming that the drive belt is slipping, but maybe it's something else. I have thoroughly cleaned out all loose grass from the drive belt (and mower attachment) and checked for anything loose. Yes, the freewheel control is engaged.
 
Also, perhaps related or not, the motor has also developed a "rattle" at the highest throttle only - stationary, mower disengaged.
 
Thx for any help, Doug B