I have a 1987 Mercruiser V6, 175 HP with a standard cooling system. Last weekend when I took it out, I heard a grinding noise that was like the bearing in the water pump of my car going bad. But in my car, the noise was consistence (idle or at higher RPM). The weird thing was the noise only started when it was in gear and running, and above 1500 RPM. There was no noise when I ran it slow (under 1500 RPM). When I put it in neutral and brought the motor up to 2000+ RPM, there was no noise. At first I thought it was the alternator, and since it was easy to take it out. I took the alternator out and spun it by hand, the bearing was good. What else could it be? Please help.
