I'm really reaching here, but ...

- are you sure your front and rear derailleurs are both properly adjusted? If not, you can likely do this yourself by following the directions at www.parktool.com

- is your chain reasonably clean and reasonably well lubricated? If not, you may be hearing some chain noise (though it usually wouldn't care what gear you're in).

- do you have a double, or a triple, chainring? Which gear combinations have you tried this in, and does changing to a smaller or larger chainring eliminate the problem?

- how many cogs do you have on the rear cassette? Same question as above (I'm trying to determine whether the "mid-range gear" is the only gear that you've climbed in. Climbing can cause frame flex, pushing your chain into your front derailleur, for example.

- can you try to give a better description of the sound? Is it more like scraping? Grinding? Clanging? Ticking? Does it happen continuously as you climb? With every revolution of the pedal? Have you heard it under any other conditions??

- What kind of bike, and what components are on it

We'll get this fixed. Meanwhile, avoid hills ... or wear a Walkman