We had a similar "whine" that eventually disappeared.. then the stuffing box drip rate increased. Repacked the stuffing box and now that whine is back at the same rpm.. so I suspect that's the source.
It's not a big issue for us as we rarely travel at that particular rpm, I just hear it briefly as we're slowing down to enter our marina. Beyond the temporary noise, the stuffing box drip rate is fine as is the running temperature.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)