Strange click every 10-20 seconds

[car5car]

Irregular click, strange sound. I can still hear it, when I don't move pedals.
I am not sure if it is coming from front or rear wheel.
I turned bike upside down and turned wheels, but couldn't hear that sound.
Can spokes make noise?
Thanks.

[Bill]

Try holding one wheel on the ground and one in the air. Move the bike back and forth a couple of feet each way for a full rotation of the wheel. Put the other wheel on the ground and do the same thing. Also what you wanna look for is cracks around the nipple of the wheel and hub.

Couple of pictures for reference, the first I am talking where the spokes attach to hub spoke holes.
http://forums.bicycletutor.com/thread-1864.html

There is an older thread I could not find here but here is one if you look closely the wheel has a crack/ripped metal on both sides of the nipple.

http://forums.bicycletutor.com/thread-762.html

The only other thing I could think of is maybe your rear derailler is not kicking in gear correctly. turning it upside down may give a different angle enough to stop the noise.

[nfmisso]

Yes spokes can make noise. Try plucking each spoke in both wheels, they should all have about the same note, and not a thud.