Brand new prop from Bukh would "sing" a most intrusive note between 1800 and 2200 revs.
Today put boat on slipway and washed the hull but also followed the advice of
a marine engineer regarding the prop.
He told me to measure the thickness of the propellor half way around one blade
(which turned out to be 3mm) and then on one edge only of each blade,
file off one third of the thickness ( in my case 1mm) at an angle of 45 degrees.
It has worked. Completely elliminated the noise.