thanks to everyone for their input, especially Joe60. i especially found the technical explanations useful and interesting to follow.
i have arranged with the RGI plumber for him to install a new fan. he says it's not a big job, no more than two hours.
sorry for not supplying more boiler...
I don't think it is an ecotec. We got it just before / as the condensing option was available. Also, I think the plumber is a vaillant certified plumber but I just wanted to understand and feel a tad more eductated in getting to grips with my boiler/ch system. Thanks everyone. I'm listening!!!
Oh my. I am following this with great interest - being one of those people who likes to UNDERSTAND THINGS. Anyway, this is what I am getting: it is probably the fan but if that does not work (it's a two hour job apparently) then it could be the heat exchanger. My, it's chatty here....
I have a Vaillant combination boiler about ten years old. Since having an expansion vessel fitted recently, the boiler makes a noise like a plane going over when the hot water is running.
The noise starts soon after the hot water is running and goes down in tone lasting up to thirty seconds...