I have a 2007 yamaha 90hp 4 stroke. Bought the boat last year (new) and now has approx 50 hrs on it. The problem is when I idle down and travel at low speeds it makes a high pitch whine. It will stay that way untill I speed up. The pump seems fine and and so does the fly-wheel belt, there was a slight rubbing on the cover. I had a mechanic check it out but he didn't know what it was. This has done this since new. Does anyone know what it might be ?