Well, call it blasphemy but I still say that belt primary tensions are being recommended too loose
Norvil says set it up so there is a 90 degree twist in the upper belt in the middle, this advice is now considered sacred
that is too loose and it comes from someone originally and rightfully understanding that as the baskets and primary heats up
the belt of course gets tighter
go ahead, set it up with a 90 degree twist, ride until the primary is blazing hot, pull off the outer cover and you still have plenty
of belt looseness
remember, the belt is tightest at the top because of the drive pull, the bottom belt will signal too tight only it has virtually no slack
with a fully heated up primary
the sound you are hearing is the belt slapping the bottom on the primary because it is too damn loose and will now wear faster too
set the belt when cold with around 70 degrees twist not 90 or some 30mm not 35mm upper belt movement
downward pressure on the kickstarter will tighten the upper belt, doing so with a fully hot primary will show you that the lower belt still has some slack, perfect