96 Merc 150hp continuous beep at high revs.

[steveoli]

I have 2 x 1996 150 mercs on the back of my Bayliner trophy 2509, I had a constant alarm sounding when I reach high revs. But before I had a chance to take it down to the dealership I now get a constant beep as soon as I trim down the engine into the water. I thought it was the impellers and an overheating problems but now with this new fault I have my doubts. Any previous fault diagnosed from anyone. Much appreciated help thanks.

 

[steveoli]

I would like to add that the engine runs well all the time.