i have a 96 scrambler 400 everything works great, but when i really push her hard i get a snapping sound, just 1 snap, like if the chain was loose and it skipped on a worn sprocket. all sprockets are great shape and chains are tight as they should be. i noticed it once when going through a big water hole that was full of big pieces of ice (the quad ahead of me broke throught the ice), and then once when i tried to get her to spin the rear tires from a dead stop on dry dirt road.
under normal use she works fine even when gassing hard.

 

Could be a rear axle bearing making the noise or an internal "silent" chain in the gear case that's worn and jumping a tooth when pushed hard.Put the front end against a pole and give it throttle without breaking the tires loose and see it it does this.

 

It could also be the plastic chain guide that mounts to a welded on tab on the swingarm. The chain guide has a bolt in it that allows you to move it forwards or backwards when you loosen it when adjusting the chain slack. The bold may be loose or missing.

 

thanks for the replies, for now its still working good under normal use so im not gonna tear it apart yet though. cheers.
heres a pic of my last polaris, a 91 indy trail deluxe, the only machine i ever bought brand new

 

Kinda reminds me of the old 92-93 ? Storm that was the only snowmobile I ever worked on. Don't see too many of em down here. I'd clean the owner's carbs when he forgot to store it up and get it ready for Colorado.

 

oh yea i drove a buddys storm 800. 120mph was the last i saw as i glanced at the speedo going across the lake. crazy.