Having your chain well oiled does a lot to reduce drive train noise. My bike was REALLY loud until I put on Phil's Tenacious Oil. Now basically all I can hear is the wheels on the road.

Also I'd imagine that using a 1/8" chain on a 3/32" cog/chainring might be somewhat noisier than having matching sizes. I have a 1/8" chain and cog, but a 3/32" chainring and mine's quiet though.