I may have missed what engine/drive combination you have, but if it is a Yanmar
with SD20 saildrive
it could easily be singing from a wrong installation
. We had this on both of our replaced engines, turned out to be that the fixed pitch
props were not installed with a tail cone rather than a nut.
Talking to prop people at a boat show
last week, this occurred because the saildrives are shipped with a nut for a folding prop as standard. If you fit fixed props you need tailcones, its in the installation
instructions but pros never read them.