If you check the chain in the high gears, you'll see that the chain is rubbing on the front deraileur. That's because it is set way to high by Kent when the bike is put together. So move the deraileur down to were it well just clear the big chainring when you down shift. That will stop the "rattling sound" and keep the chain from wearing out so fast.

I movied mine down about 3/8 of a inch down.