My 98 FIII 600 used to do something like you describe, mine would "squeal" under acceleration, and at sustained high trail speeds. After some investigation I found a few things, but what fixed the problem was clutches out of alignment. I was trying to set it with a straightedge and a ruler.....big mistake, I bought one of those aluminim ones made specifically for my sled for $20 from SLP and voila noise gone. I will never go without the proper clutch alignment too again.
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