2007 37 fitted with the standard 2 blade aluminium prop. It is the prop. There is a Volvo bulletin issued to dealers about changing the edge of the prop. I had mine done by Volspec and the noise disappeared.
Having now had the coating come off leaving a raw paint edge, the noise is back. I’m pretty sure it has nothing to do with the gearbox or oil as I have never heard of a gearbox failing. See another recent thread by me – ATF was specified for the 2007 D1-30. It is now 10w 40 engine oil due to possible clutch problems. ATF and engine oil do mix I am told. Some ATF gets left in the box when changing to 10W 40.
I’ve had this confirmed by 2 Volvo agents.
If you put your ear to the hull bottom or aft cabin sole you can clearly hear the whine but not with your head in the engine compartment.