I have abnormally sharp hearing, so I can hear every bit of noise my bike makes -- and often the noises of bikes riding past me.
But I'm not a bike mechanic -- so sometimes when I identify a sound I don't know if it's a problem, or falls into the category of "yeah, they all sound like that."
For example, I can hear the noise of my chain engaging the rear cog. It's a slightly metallic, almost grinding noise, just slightly louder than the hum of my tires against the pavement. I'm pretty sure the chain is neither too tight nor too loose, no stuck links, and the chainline seems as straight as I can make it.
Okay... so other than taking this to one or more LBS and asking them to listen, or borrowing/test riding a bunch of other bikes for sound comparisons -- how do I know what a normal and healthy FG chain should sound like?