Mercury 2000 150 EFI - Intermittent solid beep alarm

Help please?.I?m getting an intermittent solid beep alarms on both engine at random times ? however mostly on the port engine. I have replaced both thermostats on both engines. I replace the water separator filters on both engines and I still get the Intermittent solid ?beep? alarm/s. It seems to be heat related since I get the alarms after running as 3000 rpm for about 15 minutes. If I go to idle for 5 minutes, the alarm goes off. I engines don?t seem to be running hot, but not sure.
Also, what is the LED on the alarm sensor (black unit / box in the front of the engine) for and is there a code flashed? (I have not run with the covers off ? yet.)
What is the best way to debug this problem?
Any help would be greatly appreciated.
Thanks?..

Re: Mercury 2000 150 EFI - Intermittent solid beep alarm

A solid tone is a temp alarm. How old are the water pumps and impellers, how strong are the 'P' streams, do you have water pressure guages to keep track of cooling pressure?