I have recently been converting a Seaport Workboat to freelance steam outline tug. An expensive sound unit would be a bit OTT for such a simple model and I have been experimenting with a cheap MP3 player and amplifier/speaker unit from ASDA at £10 and £5 respectively. I have downloaded an engine noise and various other mechanical sounds from the web. I replicated the basic engine noise several times over and then randomly overlayed various bangs, clangs, hammering, squeaking noises etc and saved out to an MP3 file which I transferred to the player.
There are obviously lots of snags and disadvantages, e.g. the noise is not coupled to the throttle, and the whole thing is a bit Mickey Mouse but an interesting addition to a cheapo model. I take the points made about what one can and can't hear from a ship in motion but often we are more comfortable with what we 'expect' to hear and see rather than reality?
Interesting thread which has prompted me to make my first posting.