Worked on a twin screw ocean going vessel for three years. Used to go down in the engine room underway and have a chat with the duty engineer. No ear muffs were required, remember the main noise was the low rumbling of the big ends. In the boiler room, the noise were the air fans and the regular beat of the feed pump, but when they blew down the boiler because of feed contamination, wow what a noise sounded a bit like a dragoon roaring. When I stood on the flying bridge underway, all you heard was a constant roar of combustion gas and water gurgling out of the steam whistle. I do not know if that helps, a friend of mine left a tape recorder going all day in the Thames tug challenger, he moaned that from a occasional low hiss of steam, was engine room telegraph movement and the feed pump. Now railway locos, yes they do make wonderful noise as thier exhaust steam is blasted straight up the chimney to atmosphere, unlike marine steam where the steam is captured back into the feed water tank.