Need help identifying rattle
 

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Far and away the most common cause is the waste end of the FD cable being close to the frame and tapping the frame on bumps.

Chain slap?

Check the nut on your tire tube. It comes loose and rattles all the time on my bikes.

I have this happen with my front wheel all the time. Drives me crazy when I head out and get a little ways into my ride and hit a bump and hear it. Not a huge deal to stop and fix except for my crappy cyclecomputer which is a pain to get reading the magnet again.

If I remember to, I usually bounce the front end a little in the driveway before I head out to make sure it isn't loose.

To clarify, "tire tube" sounds like the tube inside the tire. The axle nut is what is being referred. The nut holding the axle and bearings together, not the quick release.

This. I do some off road touring and it happens.

Also, if you have a saddle bag, check to see if things are loose enough to rattle around, including CO2 cartridges.

Had someone come into the shop yesterday with a similar problem. A nipple on the front wheel had come loose and was running wild inside her rim.

Do your cables run in or on the outside of your frame? My bike is also making noise, and I'm betting it is the cable rattling in the frame.

Loose headset?

Pump, bottle cages, items mentioned above, lights, cables slapping, missing cable doohingeys letting the cables slap.

If it is a noise, then it is likely caused by some accessory like a pump, rack, fenders, water bottle cage, etc. If it is more of a rattle you 'feel' - like a keychain rattling around in a metal bucket, then it could be loose bearings like a headset, or hubs. Chainslap is another prime suspect.

Something in the seat tube?