Your description sounds like your chain is hitting your front derailleur and here is why I say this. As you go up in gear (to the outside and smallest cog) the chain moves away from the bike frame and more toward the outside of the derailleur cage and because of this the noise gets louder. As for why your noise gets less frequent, my guess is that the because you cant pedal as fast in the higher gears as you can in the lower gears the chain isnt rubbing the front derailleur as quickly therefore you dont hear it as often.
Advice: read about the type of front derailleur you have on sheldonbrown.com or parktool.com and try to adjust yourself. If you dont feel confidant enough to give it a shot, take it to a bike shop. However, you will learn way more if you do it yourself and the next time something happens you will at least know you can go at your bike with a wrench without it blowing up
. If this doesnt fix it, there are many many other things that could cause this noise, as I am well aware. The front derailleur is just my first guess. good luck.