A tight belt or too tight will cause a chirping sounds. My bike did that after I had my tires changed out. Is the oil level good?
If you catch it early enough before it does damage to the output shaft bearing you can save a few bucks. Mine was too loose for too long and the output shaft bearing failed. When I took it to the dealer for the noise they told me 900's make a lot of noise, just ride it. It only got worse.Just tightening that nut is not good enough. The pulley should be removed and the splines of the pulley and output shaft inspected for wear and slop.
The nut is there to keep the pulley from falling off only and does not tighten the pulley on the shaft. Tightening that nut gut-busting tight will not remedy the underlying fault. if the nut is loose, there is a reason for it.