I checked with an ex Grey Funnel line a while ago - didn't know which vessels to which they were fitted.
Anyway His Admiralship said they were steam powered sirens - always jamming up so a junior rate on report would be given the task of cleaning it. I think that the water content gave the a certain "quality" that compressed air cannot.
I've not been able to find anything advertised. However, watching the (Universities) Boat Race a couple of years ago, I swore I heard that distinctive sound. As there were no vessels of appropriate size at that location (Barnes, S.W. London), let alone a steamer, I suppose it could have been electronic?
No they were steam powered. Often mounted either side of the funnel. You stayed off the upper deck when they were sounding /forums/images/graemlins/grin.gif
I bought a Klaxon from Nordic_Ranger and it's just had a new horn made and been fitted to Sanfiona, the double-diag workboat that you were admiring being re-built.