so i have a 1.5 month old Motobecane Le Champion Ti SL that has developed a double click on the down stroke of my left crank. the click is inexplicably intermittent and it is ONLY heard when i'm exerting a decent amount of force on the pedals (20+ mph); its' never heard at lower speeds.
i took it in to a LBS and the wrench there said it was a loose B/B cup, so he tightened it up and the noise went away for 4 days of beautifully silent commuting, and i was happy. then the clicking came back. i'm hesitant to take it back to the same shop because they didn't solve the problem the first time and i'm not inclined to spend more money to have the problem not fixed again.
so for all of you bike wizards out there, are there any likely suspects that i should look at first to diagnose where this mystery click is coming from? i know it's not chain or derailleur related, it's got to be somewhere in the crank or pedals, but the thing making it so hard to find is that the sound can only be heard when i'm riding the bike fairly hard, when i spin it in the stand, all is silent and well.
P.S. there's no need to point out that i got what i paid for by going the BD route, i'm already well aware of the opinions of anti-BD crowd.