I have a problem of a knocking while I'm pedaling. Pedal or crank arm is not hitting frame. Pedal is tight. Crank arm is tight. It does knock at the same point of pedal stroke (left pedal straight up). I can actually sort of feel it in the bottom of my shoe when pedaling. It sounds like when I was a kid and the pedal hit the kickstand that was not put up all the way. Difference being it does not make the knocking sound on every stroke. Just most. Thoughts, suggestions, ideas on what this could be would be much appreciated.