Q, what type of hot-water system do you have, a hot-water cylinder? or a combi-boiler?
if it is the later, what you are hearing is due to you wanting 'extra hot' baths / showers, and the main water supply being even colder than usual, and the resultant extra work the boiler is trying to do, by pushing more water through the heat exchanger than normal. heat exchangers have narrow passages, this can further be restricted to provide more heat. this is what I reckon it is. ;)