So our SL (~16K miles) also makes the same "screeching" noise when under load (acceleration). Totally quiet during coasting or on the downhill slopes. So I gave the drive-belt an extreme cleaning (using a good de-greasing detergent soap and a wire brush, thoroughly scrubbing each of the "Teeth and Valleys" on the belt, as well as the belt's side-walls.
First drive after cleaning: No more screeching! Yea, Success!
HOWEVER ... within ~5 miles of driving (normal street driving, not on a dirty or dusty road) the screeching noise started coming back. low volume at first, but over time gradually increasing until it was the same as before the cleaning.
So I am convinced the belt cleanliness is a contributing factor in the screeching noise, but it is not the only reason for the noise. If cleanliness was the only reason, who has time to thoroughly clean the belt before each drive in hopes the noise wont come back again while riding?
Note: for reference, because I'm a big guy, I had my belt tension adjusted by my dealer to a little looser than factory spec (~3/4" to 1" deflection at 10 lbs). And the belt alignment (while stopped) looks like it should be: to the left, I.E. nearer to the wheel side of the sprocket, but not actually touching the sprocket side wall.
Anecdotally: it seems the screeching is more noticeable when there are two riders in the SS (more weight = more belt tension?). Also, I have not seen any correlation between noise level and the ambient temperature. During a recent 500 mile day trip that covered from ~45 degrees to 95 degrees I did not hear any significant variation in noise volume dependent on the temperature.
My belt never made this noise when the SS was newer, and before the swing-arm replacement was done (at around 9K miles). I'm stymied as to what to do at this point, short of paying the dealer (again) to check/change the belt alignment. Even if its initially "fixed" by a re-alignment, I'm not convinced it wont come back again. I wish one of us with this issue, working with their Polaris service department could identify a conclusive fix for this screeching noise.
For now, I'm just cranking up the radio volume so I don't have to hear it so much, but it sure is annoying to have to put up with this screeching noise (and a little embarrassing when folks hear it when I'm pulling away from them)