Re-posting of my previous reply:
OK, I'll share my experience and whats working for our 2016 SS SL
After ~2+ years/~15K miles of quiet riding, we also started hearing the horrendous shrieking/whistle noise from the drive belt. But only during acceleration/loading the belt.
(1) I did a very thorough belt cleaning with a wire brush and a strong de-greaser. Noise stopped initially, but started coming back within ~5 miles of riding (ordinary street driving, not in "dirty" conditions)
(2) I had the belt tension and alignment re-set by my Polaris service dept. but this did not solve the problem (had the proper gap on the left side of the belt, and tension was set towards the loose side of the spec, ~.75") The dealer was unsure what to do next, basically said to "... just live with it. Slingshots make all kinds of funny noises". I also saw in a related post someone mentioned their right angle drive was "adjusted" by the their dealer and that fixed the noise. My dealer confirmed their is no adjustment to be made to "re-align" the right angle drive, it can only be adjusted for the belt tension, and the pivot shaft adjusted for belt alignment.
(3) During a recent 500+ mile drive-day, I experienced temperatures ranging from Low 40's (Tioga pass/Yosemite) and up to 98 degrees (Death Valley), and the noise was just the same whatever the temperature.
So short of buying a new belt just to see what that might sound like, I started thinking of trying an application of some "substance" to the belt to see if that would help (Like the ideas of the "lubricant" or graphite).
Then a thought occurred to me that some of the older "shade tree" mechanics might recognize. Back in the day, what was a temporary quick-fix for a noisy-squealing fan belt? ..... Holding a bar of soap against the contact surface of the fan belt while the engine was idling.
So... I jacked up the back end of the SS so the wheel could turn freely, and I liberally rubbed a bar of soap (Ivory) against the 3 sides of the belt while turning the wheel by hand ( on the two side edges and the belt's teeth).
Son of a gun if the belt didn't quiet right up! And several hundred miles later it still was running quiet. I know this is not a permanent fix, but when the noise starts up again I'll just repeat the soap rub. (Although I do wonder if I'm caught in the rain if I'll be leaving a trail of soap bubbles behind me ? )
Update: I used a white bar of soap at first because it was handy, and just to try out the idea. It does leave some unsightly white residue on the black belt, but I was willing to accept the residue just to kill the noise. My better solution: my wife found a Black colored bar of soap that seems to work just as well, but doesn't show up against the belt so much.
Good luck if you try it out on your ride. This works for me, but as the saying goes, "your mileage may vary ..."