Advice from troubleshooting violin buzzes: noises like buzzes and clicks can originate in one place but then the vibration will travel quite a distance until it finds some other piece that resonates at the frequency of the noise to act like a speaker. On one cello I found the problem was a loose screw more than two feet away from where the buzz was becoming sound. So a top of seatpost source for a noise in the bottom bracket doesn't sound like an unfamiliar idea at all to me.