My caravan boiler, Britony Combi SE, makes a noise like a passing light aircraft when a hot tap or shower is switched on (runs absolutely fine when only central heating is on). The problem has become worse over the course of this season and can now be clearly heard outside the caravan. So far the site plumbers/contractors have failed to solve the problem after fitting a new thermister, they now wish to change the PCB (total cost £165) but I have my doubts especially when the noise appears to be a high frequency vibration. The boiler performs extremely well apart from the noise and I can see no flashing LEDs as mentioned in the maintainance guide. Has anyone come across this problem and if so what finally cured it. I am about to fork out for manufacturer recommended plumbers but my experience so far does not fill me with confidence (previous plumbers have all been corgi registered!!) Finally the noise appears to come from the heat exchanger area but I have definately found no problem with the fan assembly. The caravan is 5 years old.