I was on my way to a meeting, pedaling along, when suddenly there was a loud creak. I stopped, visually inspected the bike, manually revolved cranks both ways, and made sure nothing was sticking into any revolving parts like spokes.
I couldn't find anything wrong so I mounted the bike again and started pedaling carefully. Then it made the sound again about 50 meters later. This time, I'm fair sure it was from the bottom bracket area, but of course, you can never know for sure.
I canceled my meeting and currently I'm walking my bike around looking for a LBS that is open this hour. I'm not carrying enough tools to go loose on the BB, chain or cranks.
My bicycle is a 1970s steel Benotto with a Shimano 600 crankset.
Anyone have any idea what this could be? Feel free to ask questions. I won't have anything to do for a while now...
I have tried applying force to various parts of the frame and wheels without being able to trigger the sound, but for all I know I may have missed the important part. So far, I gave only been able to trigger the sound by sitting on my bike and pedaling (both carefully and violently.) I have not heard the sound when just coasting, but on the other hand I have also not spent a significant amount of time coasting since it happened (if we exclude walking with the bike next to me, which I have done quite a bit, by now.)
Seat post is another possibility, but I haven't been able to reproduce the sound when stationary yet, even if I shift my weight around on the saddle.
Edit: new evidence! I sat up and thought I'd try riding a bit toward my destination. After a couple of minutes, the pedals started resisting way more than they should in this gear. Both backwards and forwards. Coasting still no problem.