Diagnosing buzzing sound when riding over uneven terrain I've got an old steel-frame road bike which has started making a very unpleasant and worrying buzzing sound when riding over rough/uneven terrain (e.g. roads with lots of fissures/cracks, cobblestones etc.). I've tried


(lightly) whacking the metal fenders with my hand
(lightly) dropping the bike on each wheel individually as well as together
shaking the bike vigorously 
and hitting/spinning the quill pedals


but I can't seem to reproduce the sound when not riding. What might it be? What steps can I take to diagnose this?
Stop riding it immediately!

Remove both wheels and feel the rims for cracks.  Tap lightly all the spokes looking for broken ones.  Check the wheel for trueness.  Grab pairs of spokes and give them a squeeze.  You're looking for damage.

If both wheels seem OK then wash the bike and look for cracks under a strong light.  Look all round both the chainstays and the seat stays.  Since the wheels are off, you can look at the inside of the stays.  Then examine the inside of the fork too.  Finally look over the main tubes.

If you find cracks, take photos and post them here.



If you find no cracks, reassemble and see if the noise has changed.  If you have access to rollers, or a quiet road, try and reproduce the noise.  Try variations like this:


Coast while standing on the pedals - does the noise increase/decrease?
Pedal while standing - does the noise increase/decrease?
Pedal backwards - does the noise increase/decrease?
Put more weight on the back wheel (ie, put your backside backward, and your belly on the saddle - does the noise increase/decrease?
Put your weight well forward (that is, crotch to stem, like a steep climb)  does the noise increase/decrease?
Lean the bike hard while riding - put the saddle or top tube against your inside knee.  Is there any difference leaning to the left vs the right?


Quiet street required because you're not completely focussed on the road around you.   Use your best judgement.

If you have a riding partner consider asking them to listen and examine your bike while riding slowly together.

A process of careful elimination might help isolate what area of the bike is the cause.