Proper alignment of the rear wheel and drive belt is easy to do... that's why there are adjusters on both side of the axle... measure from the middle of the swing arm pivot bolt to the middle of the axle nut on both sides... if it is the same then the alignment is correct. If it is not the same, then adjust it until it is. The belt should run around the pulley going forward approximately centered... by the way, do not use belt dressing on the belt.

 

Yeah all belts move from side to side with a forward or reverse motion.

 

I can't say for sure without hearing the sound, but I know I had a similar noise on decel last year. Went right to dealer and he said "don't worry, that's normal" I thought, yeah right, something is big time wrong here, but left it alone, thinking at least i have warranty when something finally lets go. Anyways, a few thousand miles later, it went away and never came back. How many miles do you have on your bike? Oh, and no it wasn't there when I first bought the bike. It just kinda came and went. Sry if that's not much help.