Rattling sound on gears 7 and 8???
 

I have a 24-speed mountain bike. When I shift the smaller gears to 7 or 8 there is a rattling sound, somethink like a chain hitting a chain whenever I pedal. This is a new bike. Any thoughts?

I'm not sure what you mean by smaller gears. Basically, if you are on the large chainring and the problem occurs on the large cassette cogs or, if you are on the small chainring and the small cassette cogs, then the chain is making contact with the front derailleur cage. Just move to the middle chain ring and choose another combination of gears that will work. It is not good to use the gear combinations that have the chain angled between large and large or small and small. If I misunderstand the problem, then it is an adjustment issue.

It sounds like you need to adjust your front derailleur. When you go to 7-8th gear (rear cassette), the sound you hear is the chain rubbing against the FD cage.

You mention it's a new bike... which has new shifter cables...

Well, one of the joys of the cycling life is that when you have new cables (gear or brake) you get "stretch" in the beginning.

Yes... cross-chaining will cause this problem, and should be avoided at all costs.
But I bet this isn't the posters problem.

New Bike. New Cables. Rubbing on the extremes...

Cable stretch.

A derailleur (and prolly brake) adj would be my first approach.
TAKE UP THAT SLACK!

I think you're all right on this one. The LBS guy did tell me about stretching. And he told me to take the bike in when it's stretched. I didn't know stretching would cause my problem. Is this something a newbie can attempt to fix?

You could attempt it but why if it's a new bike and the mech asked you to bring it in for a free adjustment? best thing would be to take it in and politely ask if you can watch and have the procedure briefly explained as s/he does it. Be honest, don't be afraid to say that you don't know how.

I have found that if they're not too busy and they can do the adjustment on the spot then generally the better shops (customer focused) don't mind showing you.

I had a really good experience with the bike I have now, on it's first adjustment I asked a few adjustment type quertions about the rear derailleur so the mech invited me to hang out in the workshop while he did the job and he willingly passed on dozens of tips on a whole bunch of things, he's a real people person so it's probably the exception rather than the rule. Ofcourse, he ensured that when I need something he gets first go at getting my business every time..... works both ways.

fretman-

Interesting! I'm new to a Trek 820 mtb, 21 speed and I get a "rumbling" periodically in the big chainring and smallest gears, so the 3-6 and 3-7 combinations

It "feels" like bad bearings in the crank to me as a noobe - but LBS dealer says the way those roller bearings are made it couldn't be there - has to be somewhere else

More of a curiosity than an annoyance - I don't think I've noticed it lately either

Peter

If it was the crank, then it would happen in all your gears.
A chain rubbing against the front der. cage will feel/sound like "rumbling"

Thanks to all for the great advice. I'll take my bike into my LBS soon. I have free tune-ups for 2 years.