It's gonna do that.

If it doesn't squeal while riding, then I think you're in good shape. Not always, but a noisy squeaking/whining belt under operation means a potentially misaligned rear wheel or pulley.

My belt makes a noise under decel. I think it's normal. It may be a little tight though.

Even with the rear shim mod to make sure everything was aligned, mine started to squeal a few months ago. I had my belt tensioned to the 90 degree two finger method. However, I tried loosening the belt just a smidgen and whatta you know, the squeal stopped!  So, don't go too loose, but if you have the patience you can consider lightening the tension just a bit and see if that helps. As I understand it, when you sit on the bike and depress the shocks it increases the tension on the belt so a smidgen loose at rest is not too critical so long as you weigh more than 100 lbs and actually depress the shocks when you sit on the bike.

Dave/Versy/Serow (and others), please correct me if I am leading anyone astray here.

I had a chirpy belt, turned out it was this:

The teeth had worn from a square profile to a triangle profile allowing the pulley to wobble on the shaft. The belt rubbing on the out of alignment pulley was only a symptom.

Yes, solved with a new pulley. No wobble, no chirp!

If you suspect this is the cause of your chirp, put one hand on each side of the pulley and twist back and forth. Any movement is a bad sign.

Didn't OF "glue" his down with RTV or am I thinking of something else?

Oldfeller, sorry to refer to you in the 3rd person. I don't see you around much.