Too tight puts tension on the trans output shaft, gets it slightly out of square, lets gears make contact even with the clutch in or the bike in neutral. The vibration/sound from the contact, however slight, can be transmitted through the clutch basket (which is tied into the input shaft and therefor the gears).

Confused yet? Unit construction bikes like most metrics and all Sportsters are kind of, um, odd... compared to a car/truck transmission or pre-unit motorcycles (Big Twins, early 60s and older British, Indians)... but still less finicky about how they are set up. Try to tighten the primary chain on an OLD bike, loosen a bunch of bolts, slide trans back and tighten the bolts, then reset your drivechain tension, all while keeping the primary sprocket, countershaft sprocket, and driven sprocket in perfect alignment... triple row primary chains aren't cheap.

WD, the first paragraph makes sense for sure. in fact, I think that's probably what's been happening. when i got the bike the belt was too tight, but i didn't hear any zzzzz noise untill the belt was too loose. Which in my mind makes sense.

When I propped the bike up, removed the cover over the primary and spun the belt i can see that the prmary pully (or whatever its called) does have some sort of lope or or something to it but you cant grab a hold of it and move it "out of square" like the rear pully. I'll have to prop it up again and study the primary.

huh... sh*t i dont know. I rode down to Laguna and back today. I noticed on my way back it was making that zzzzz sound again on deceleration with the clutch out. Whatever... the bike hauls a*s. At this point as long as there is no serious or permanent damage going on i cant be bothered to care. I'm not sure I would know what to do anyway. I loosen the belt it makes a sound,, I tighten the belt and makes a sound. There is a new noise every time I get on that bike. Psh, whatever