Overheat Alarm????

I just bought a 2000 Bennington Pontoon with a 2000 50HP ELPTO Mercury. The guy told me that the overheat alarm would go off if you idled the motor for an extended period. I wasn't sure what the alarm sounded like but I hear a screeching noise coming from the motor at certain throttle positions. I would say at a low idle, maybe 10% throttle it does NOT do it. But from about 20-50% throttle it does DO it. But from 50% on up it does NOT do it. It seems to make the noise no matter if the motor is cold or warm. The water hole has a steady stream peeing out of it all the time. My brother in law said it should be shooting out of the hole. I took the cover off and took off the hose and it was clean. Is it the overheat alarm going off, why would it do it when the motor is first started? If it is, how can I test it? I also read in another post about the vanes in the water pump, where are they at so I can check them or replace them? Any help would be greatly appreciated. Thank You in advance - Mike