Has anyone experienced their drive belt sqeaking or chirping on deceleration? I've cleaned the belt and pulley, checked adjustments, checked clearances, ect. It is an aftermaket aluminum pulley, it's the correct size and has the correct number of teeth. Worked fine for over a year and now it makes this noise, but only when you let off the throttle. It doesn't do it when you pull the clutch in, either. ( that was checked also; everything O.K.) We've even spayed the belt while the tire was running (on the dyno) and the noise stopped instantly, but starts back after a few miles. Any information would be greatly appreciated. 
I had a very similar issue but I did not try the spray as you did so I cannot speak to that. Mine would chirp as soon as I would let off the throttle. On the lift, it would chirp when the bike was in gear and you rocked the wheel back and forth. During freewheel, it would make a low creaking noise. My mindset is to always start with the easy, cheap and usual issues, here's what I did...
1) Adjusted belt - Nope
2) Cleaned belt and pulley - Nope
3) Cleaned compensator isolators - Nope
4) Checked alignment each time I did the steps above - Nope
5) Inspected pulley with a bright light - Ah ha! Two hairline cracks I could only see at an angle. OBTW, they're hollow.
6) Replaced pulley - Problem solved
Hope this helps someone searching for an answer to what they can't seem to find after doing some of the steps above. It wasn't the answer I wanted, but it was what I ended with.