Finding a rattle in my bike
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.