Make sure your wheel is aligned properly, and your chain tension is good. Your first one, with vertical dropouts? these 2 items can certainly be an issue. And buy a bottle of good chain lube. Don't spray WD-40 on all your problems! There will always be some noise: there are parts of the chain continously colliding with parts of the chainring/cog. The most obvious manifestation of this is noise. The cog tends to be louder since the chain makes a tighter turn here than it does on the chainring.