Bicycle Mechanics - Gear noise

I am a newbie to the world of Cycling. I recently bought a Trek, and my girlfriend purchased a Mongoose. She has noticed a slight rattling in certain gears as if the bike is not quite in the correct gear and wants to change to another.

Any ideas on what this might be?

Thanks for any help

If you buy a bike at a LBS, they generally want to check the bike out after a month or two, depending on how much you ride it. This check allows them (amongst other things) to adjust the shifting to account for the cable stretch that has affected the shifting quality of your GF's bike. Since she bought a dept. store bike at a big box store, I doubt that anyone there would know what to do if you did bring the bike back. Look for a knurled barrel on the rear derailleur where the shift cable exits the deraileur and loops up and across the chainstay. Adjust this barrel counter-clockwise, by small amounts (1/8 of a turn) until the noise goes away.

Great, thanks for the advice. i will try that out.