Adjust your chain tension so that at its tightest point you can lift the bottom of the chain at its middle about 1/4". Make sure the rear wheel is centered in the frame. Also, check for any tight or kinked links in the chain. Lubricate the chain. If the noise persists, it could mean a mis-match of components leading to a bad chain line or possibly a really cheap rear cog with mis-aligned teeth. A 3/32" chain will be more tolerant of mis-alignment, because it is designed to flex sideways on multi-gear systems, whereas a 1/8" chain is not.