we have the sportsman 700, it also had the rattle. Check your exhaust bolts, that is where the noise was coming from on ours.  

i have a 2001 700 u also have to remember that the timing in the polaris is gear driven u will here some noise.  

polaris makes one of the best rides out their they invited yamaha to a contest along with the rest of the bike manufactures  suzuki was the only one to except the invite.  

My 2008 Polaris 800 does the same thing, I had the polaris REP come to my house and look at it. The noise that is....I had it on jack stands and was able to hear the noise.  Polaris REP said that he would get back to me and thought it was the clutch engaging and disengagin with the trasmission, white traveling down or on flat hill, (on and off the throttle).  He said that they are just built that way, but would get back to me if anything could be done about the noise.  

The noise that you hear in the sportsman drivetrain when decelerating or release the throttle on down hill or changing direction does resemble that of a worn universal joint. The noise is a result of the backlash on th tranny and diff.
It is inherent to the design and does does not compromise reliability.  As for the Polaris bashers well I will let the sales numbers and growing market share speak for itself  

yeah while the recall record for polaris atv's speaks volumes for the gross lack of quality.
yeah the slack in the differential and tranny are a big problem for polaris. if you have gears slamming into each other constantly, eventually they will either break teeth or wear out prematurely.

Polaris knows this but so far they are making it just past the warranty on the machine without having to be replaced so Polaris won't admit to the defect.
But Rather POLARIS passes the cost for repairs onto their customers.
you can always rely on Polaris to stiff their customers when it comes to warranty and fixing their junk atv's.

the recalls for polaris speaks volumes you can read them here.
http://www.atvsafety.gov/atvrecalls.html