I hear this all the time I believe it is normal it sound like the driveshaft in the rear uV joints from going over big bumps. If you keep on the gas you will notice its not there.

But then again what do I know about 4x4quads not much.

I will tell you that Vinson rides great I use it at the farm auto is great when your getting off and on all the time.

 

My friend has the same complaint with his manual. I do notice a noise when I let off the throttle and then get back on it but I think it just normal drive train noise from the quick change from a no load to loaded condition.

 

Check your upper motor mount. They are prone to cracking and will make a noise like you discribed.