Another note, if you have absolutely no idea what it is, start with the simple stuff. The chain is the first part to look at. If it's stiff or worn out, it's a cheap fix. Check your lock ring for integrity, check that your spokes are at proper tension, check for cracks in the frame, check that your pedal bearings are in good order, check that it's not your shoelace or something hitting the frame (this also caused a random clicking sound for me), check that your seat post is installed correctly. Take the seat post out, clean it off, and put a light film of grease on before reinstalling it. I've had a seat post make a clicking sound before too. Check the rails on your saddle. Check your that your handlebars are in good condition, headset all tightened up properly? As you can see, there are a whole lot of stupid little parts that can make noise. I highly recommend buying a copy of "Zinn and the art of Mountain bike (or road bike, though I find the mtb one has all I need) Maintenance". It has extensive coverage of all maintenance, and a nice troubleshooting section. Good luck and let us know if you find what the problem was.