I just had a wasted exercise pulling an SD20 apart to investigate just such a
noise.
The noise could be heard in the
cockpit and
saloon, but not audible when poking my
head in the
engine hatch. I had a weeping
oil leak from the front seal. With a bit of hard running it must have leaked a bit faster due to getting hot and it was down about 250ml. I suspected I had killed the
saildrive due to a bearing running dry.
So with the
boat hauled out I had the
Yanmar specialists remove and strip it apart. They found nothing wrong! So back in it goes, and this time I had a closer look at the prop and compare to the other side (if only I had done so before

).
The prop on the noisy side showed a slight evidence of contact wear on the outer edge, hardly a mm deep. Fitted to the
saildrive there was plenty of clearance, so it must have only moved forwards under load.
So I took it to a prop specialist at Southport and he declared it looked fine. Nevertheless, I decided to get it rebushed which he did within a couple of hours. He also offset the bush slightly to give even more clearance. The removed bush looked perfect. He did say that the original
Volvo bush is softer than what he replaced it with.
So, the
boat is now back in the water, with me wiser and poorer and no good explanation of what was really wrong. At least the noise is gone and the weeping seal replaced.