I have had this sporadic creaking in the crank/ bottom bracket of my mountain bike. It makes said noise several times every revolution. I have brought the bike into several shops, but they have been able to do nothing. I have heard the theory that since it is an aluminum frame, it is just the bike flexing. The one thing I have noticed is that the noise stops when I ride in wet conditions. I have replaced the bottom bracket and everything is as tight as can be. Any suggestions?