Could be a gear crack. But ...
Check for one of the rods catching on a crankpin as it goes around. Since the noise is worse on curves, that doesn't seem like a cracked gear to me. I am suspecting that as the drivers slide over on the curve, a crankpin sticks out just enough for a rod to catch on it as it flies over it. Not just the rods, but also check for the crank pins that have to run behind the crosshead.
Can you run the engine REALLY slow, so you can watch the rotations of the drivers on the curves and look for this?
Does the click happen once per driver rotation? If it does, it's probably not a cracked idler gear. It could still be a crack in a driver gear, though.