Alan is correct. It's probably something....twig, pebble, bug....SOMETHING....bouncing around in a passage component.
My "go to" move for this is to remove the hose nipple fitting that screws into the side of the engine block and run the engine in a trashcan. The water force from the pump will eject anything that is trapped behind the fitting in the bypass passage.
Then, before reassembling the fitting, hose and nipples, inspect each one carefully for debris. Most people don't realize how important it is to replace that hose every few seasons as it loses it's flexibility. As it hardens, it tends to trap small items more easily. That hose HAS TO BE soft and supple to allow debris to pass through. It also has to be routed from the engine fitting up through the splash pan and out to the side cover JUST SO. If it's "misrouted" it can be kinked or pinched when the cover is put back on. It's a PAIN to get it hooked up to the cover fitting if it's routed properly.
The rule is: if it's easy to put the hose end on the cover fitting and reattach the cover, then you probably have the hose routed incorrectly. So, pay attention to how it's routed before removal but, keep in mind, it may have been misrouted by someone that was in there before you.
Good luck.
