On Saturday, February 9, 2019 at 4:34:04 PM UTC-8, jbeattie wrote:
Rule of thumb -- when you have a BB noise, start with the rear axle drop-out interface. I ignored the rule of thumb trying to track down what was clearly (in my mind) a BB problem -- a light thunk every time the left pedal arm hit the bottom of the stroke. This was on my Synapse with an OE Cannondale SI crank -- a pretty simple crank with a SRAM-like interface (bolt on arm and not pinch bolts like Shimano). So, I get out my jumbo 10mm hex socket to remove the bolt/cap, pull of the crank with a kludged Park remover (long story), check the bearings -- and they're fine. Grease everything up, re-assemble, go for a test ride in the snow, and it's still thunking. I get off the bike, rock the crank, and I can produce the same thunking -- but I don't really feel play in the crank, but I figure its moving laterally and needs another shim. Pull the crank again and put in a .5mm shim. Still thunking. Back onto the stand. The bike has a 12mm through axle rear hub (HED), which I had tightened earlier to rule out any issue there, but what I didn't notice is that the hub axle end cap had come loose, which allowed the hub to move laterally. Pull out the cone wrenches, tightened it up and thunking went away. A five minute repair. I don't know how or why the end cap loosened, but I've had the same problem on other wheels with similar thread-on end caps.

-- Jay Beattie.

I have exactly the same problem on my Basso with FSA cranks. I'll have to look into it using your advice.