The help has been stated already
!
Go pull your clutches before you really bugger something up, measure your crank run out so you know if you have a bad crank, and replace the parts that have been mentioned already, ie, buttons in the primary, and sliders in the secondary. The rattle is probably going away when you rev it because the clutches are clamping the belt and that is holding everything together under tension.
Do the work, all of this will take a couple of hours and will cost much less than one new belt! While you are in there you may as well replace your clutch springs too because if your springs are old and toasted you clutches aren't going to work fer s**t, (just go with stock unless you know you have some custom set up that you like already). Some dealers will have buttons and sliders in stock (if you are lucky), if yours doesn't you may still be able to have one of the aftermarket suppliers get them to you by next weekend but a dealer is going to take 10 days.
If you don't know how to do the work download a manual for less than $10.. and watch the Dynamo Joe videos a few times, it's easy, just go slow and be careful about aligning the parts on assembly!
Stop riding your sled until you have this figured out or it's gonna cost you a belt a day and very possibly more damage to your sled!
Oh, you don't really need the slick button retaining tools shown on DJ's video as all they do is keep the buttons compressed to make it easier to reassemble things, you can do it with your fingers it's just a bit of a pain. You can also make the compressor for the primary clutch (and secondary for that matter) out of some 5/8 all-thread and some wing nuts and washers, or a drill press works if you happen to have one... use your imagination.