Thanks for all your replies, but the saildrive oil has been changed to engine oil and i believe that the problem lies with the prop as under 1000rpm the drive is silent and over 1800rpm the same. There is no vibration at all and does 7.5kts at 1800rpm but I would like to travel slower without the high pitched note from below!! Will change the prop to a folding one and hope that will make the difference.

    Thanks again one and all.