Noise in the cruise

[Jewel]

Overall noise from engine/transmission when cruising above 1500rpm from the inner helm is substantial. (Boat is Birchwood TS37 39ft flybridge twin volvo 200hp shaft drives). Have done checks and running engines at 1500rpm but not in gear produces much less noise. Therefore most is from transmission/shafts, and especially a whistle/whine when port shaft in gear. (dont know if noise is from gearbox or shaft). As rpm. increase so does overall noise in inner helm, although whistle goes out of port shaft as revs increase above 2000rpm.
Question is would new low noise shaft couplings reduce transmission noise from engines to hull. I intended improving engineroom insulation, but now find transmission noise through the hull appears to be the cause of most noise.
Any comments.

Thanks


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