HI ,AM WORKING ON A LARGE PRIVATE YACHT, ALL OF MY HOT WATER BOILERS HAVE STARTED TO MAKE SCREAMING NOISES,CONTINUOSLY AND ALL HOURS.IS THIS A WATER PRESSURE ISSUE.
I assume you are talking about a boiler and not a calorifier
Boiling takes the form of bubbles of steam being generated on the heated surface intially followed by ebulition where steam bubbles form in the water spontaneously ( this causes the water level to rise and is sometimes referred to as swelling. If you boil a pan of water you can see this process occuring. You will also notice that the pan sings just before ebulition occurs and this is due to the steam bubbles being unstable and collapsing. When the strike the metal thy make a pinging noise. This is a very similar mechanism to cavitaion which recks your pump impellors and prop
How to cure
If this is a new thing I would look at the boiler internals to ensure there is no issue with scaling, I would check your water quality.
Without more info its difficult to say. If you want to discuss this you can email me on brian@marineengineering.org.uk