5.7l mercruiser knocking when you open throttle on 96 sea ray 240 sundeck

by Christina
(oak hill, fl)

my mother in law just purchased a 96 sea ray 240 sundeck with a 5.7l mercruiser engine. Her boyfriend test drove it 2 days before purchase and everything was fine. we took it out on the day of purchase and all was well until her boyfriend opened the throttle. it was good for about 2minutes then we heard a knocking a sound. So he slowed it down and the sound stopped. But once he sped up the sound started up again. when riding in a mininmum wake zone you hear nothing until you speed up. any idea what it could be and how to fix???

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