3. Is the noise coming from the front, middle, or rear of the bike? This can sometimes be difficult to ascertain with modern bikes as the large diameter frame tubes tend to resonate noises, making it difficult to pinpoint a source.  (Please try anyway!) 
4. Does the noise happen only when seated? Test this by pedaling both in and out of the seat. 
5. Have you washed your bike recently? Excessive use of water, especially with high-pressure sprayers, can force moisture into your components and frame and cause unusual noises. 
6. Have you had an accident recently?  On more than one occasion, Pete's found the 'mystery noise' to be a cracked frame. Eeep!
7. Does the noise only occur in certain gears? Ride in various gears while keeping the load constant and see if the noise continues.  
8. Are there any other associated issues? E.g. when the noise occurs, do your brakes rub on the rim/rotors, or do your gears slip?  
9. Do any bearings feel rough? As bearings wear out, their tolerances increase, sometimes resulting in a creaky ride.  
10. If you have a dual suspension bike, does the noise go away on the road?  It could be pivot-related if the answer is "yes".  
11. Does it only occur when climbing a steep hill at maximum effort? (This can be a tricky one to re-create at the shop because, as you know, Wright Street is pretty darn flat!)
Finally: 12. Have you recently performed any home mechanics? While many of you are more than competent enough to do the basics, the question needs to be asked.

(You can tell us. We're like a doctor.) :-)
 
We're always more than happy to try to solve these niggling and often very annoying noises, but any information that you, as the rider of the bike, can offer is always appreciated!