Finding a rattle in my bike I recently installed a rear luggage rack and washed my bike, and now I get a buzzing rattle when I ride over bumps in the road, independent of whether I happen to be pedaling at the time. It sounds like it's coming from somewhere below me as I'm riding, which probably indicates somewhere between the bottom bracket and the rear hub, or as high as the saddle. With my bike up on a repair stand I tapped the various components of the rack and tightened the screws but I didn't find anything obviously loose or that rattled as I tapped it.

How can I narrow down the source of the rattle?
Since you don't hear the noise when the bike is up on the stand, the problem is probably linked to the load of your weight on the bike (trying to formulate this in a way that doesn't sound like I'm saying you're a problem or you're overweight but it's impossible! haha). Sit on the bike, swing on the saddle to simulate the effect of a bump and have someone check if there's anything swinging/bending and touching a part of your wheel.

If you're really desperate and have some time on your hands, remove the luggage rack and go see if you hear that noise. Then re-install it carefully. Actually you might save some time doing this right off the bat.