I agree with bikeman715, especially as you refer to a knock sound. It is likely that the BB drive side is just loose enough to move under hard pedaling pressure but not enough for you to detect when checking by hand. Do take it into the shop, but one way to check is to put the pedals level, left crank forward, apply the brakes, and step down with all your weight on the pedal. Rotate 180 degrees and repeat on the right pedal. Generally that will cause the fixed cup to shift if it is loose.
Last edited by cny-bikeman; 03-08-14 at 03:38 PM.