I've been told by my dealer that its normal, due to the long length of the front drive-shaft it causes a vibration noise which sounds very concerning but doesn't cause any problems from what the dealer told me. Mine does it and mine is a 2011 Rhino 700. Problem goes away once throttle is applied. Problem is a bit more noticeable in 4WD but goes away when throttle is applied too.
I have a 2009 rhino, 160 hrs, it has that vibration, in two wheel drive it's ok, when you put it in 4 wheel drive at around 10 to 15 km/h it has a heck of a vibration, it's been there since new, make sure you are greasing the two U joints on the front drive shaft. they are a bear to get at but take the time to grease them .
The other guy had a good point, lube the driveshaft, but also try changing diff fluid and pay close attn for any shavings in the fluid. I had to rebuild my front diff due to ring gear bolts backing out.
Yes Lube the front driveshaft and it will help alot. I notice my vibration gets worse over time and then when I grease it, it gets better. But yes my 08 has the same thing, its pretty normal out of the 3 rhinos we have had.
2008 Yamaha Rhino 700 - Green
-Lexx slip on exhaust
-OD Greaseless Setup, Gold Spring
-28in Maxxis Zillas, ITP Deltas