Noise from back wheel when using 13T cog
 

My bike has developed a chattering/rattling sound when going over rough or bumpy road surfaces, but only when in the 13T cog! It appears to be coming from the area of the rear wheel. Smooth roads are OK, as are all the other cogs. I've tried to reproduce the sound at home on a bike stand, but without success. Any thoughts?

Chain slapping on the chainstay?

Eric

+1,

only on small sprockets + only on bumpy roads = chain hitting chain stay.

This is most likely, especially of it's worse when coasting vs pedaling.

Yes. ^^^^^^

Now, as to the cause.

1) Very worn out chain is now longer than it was.
2) Rear wheel is not pulled back as far as it had been in the past; either it is slipping forward or was not installed correctly.
3) Rear derailleur has swung forward.
4) Broken spring in rear derailleur tensioner.

Good luck,
Eric

Thanks for all your help. I checked over and over for chain slap, but just couldn't find where or how it was occurring. Finally, I gave up and took the bike to my local LBS. He took off the rear cassette, fiddled around for a while, and announced that he had cleared a small piece of grit from inside the hub. And with that, the problem disappeared. Why it only revealed itself while riding on the 13T cog, however, remains a mystery.

Thanks again for taking the time to respond.

The grit was allowing the cassette to move forward with the wheel, making the chain slack on the top and allowing it to hit on the stay. Because of the gear ratio, it required less friction to do this in the small cog compared to the larger ones.

I suspect the grit was wedged between the 13 tooth gear and the next gear. The grit probably prevented the chain from properly laying on the 13 tooth sprocket. The next larger sprocket held the chain a little higher so it didn't cause a problem.

Just my guess, but good work from the LBS.