I bought a small frame( on the cheap(1p) because it had a small dent near the gear lever braze ons) to use for Time Trials and fitted a MTB seatpost to give me the right saddle height. After the first few rides I noticed a clicking whilst pedalling, so I checked the bottom bracket area for play etc.... I swapped pedals over with a pair off my exsisting bike....put in a new BB still no luck it kept clicking. So I checked the freehub body - no luck there!
After putting up with it for a few weeks I decided to sell the bike to a friend in work (who knew of the noise) and buy myself a new bike. Just a couple of days before he collected it I had an idea! The frame was double butted, so I finally thought what the problem was - the seatpost was rocking in the frame at the point where the tubing was at its widest internal point!!!!!!!! So the seat post just needed trimming!!!! doh!
Even the professionals have their troglodyte moments:
I read in a bike magazine of a shop mechanic who, acting upon a customer's description of "a rythmic clicking sound when pedaling hard" removed and replaced the bottom bracket on said customer's bike...
... Only to hear the same noise when he gave the bike a test ride.
Next, he looked in the seat-bag, and found a 6" open end adjustable wrench. It had rubbed through the bag fabric, and was tapping the seatpost... "rythmically, when pedaling hard."