Can you see where the water is actually coming from? If you get in there with a flashlight you should be able to see some kind of dripping from one of the bulkheads.
Also, the water level in your overfloow looks too low, that can make it noisy. If you adjust the valve on the main overflow pipe in the sump clockwise, it should slow the flow and raise the water level.......which will in turn quiet it down (unless the water level gets too high....then the emergency overflow will start making noise as the "extra" water goes down that line)
For me, if I keep the water level about 1\2-1" below the emergency overflow pipe, it's perfectly quiet.