Why does the main burner(CF 40 furingas) start roaring at times?

Ref above my boiler is a Potterton Kingfisher free standing open flue gas fired boiler (approx 25 years old) and regularly serviced this trouble that I am having (and it is the only problem I have had with this boiler)is that it is going ok then all of a sudden it starts to roar very loudly (this does not affect my hot water supply or central heating)but it is an alarming noise,sometimes it can go for weeks without this roaring,then it starts intermitently for a couple of days,my son who is an experianced plumber has stripped it down twice checked and cleaned the main burner and serviced the boiler but this has not cured the problem,I hope you can help as my wife and myself are OAPs and can't afford a new system and are very happy with the reliabity of the Potterton Boiler which has served us very well for the past 25 years my son has also conferred with his work colleagues what they think the fault is,and they are of the opinion that it is the main burner.I hope you can shed some light on this roaring noise and if you agree with the diagnosis of my son and his workmateswho are now of the opinion that I require a new main burner (CF40 type) and where a new burner can be purchased,also a new switch unit for switching the hot water and central heating on and off as they are so stiff my wife cannot switch them on or off and I also struggle myself at times,any help and advice on these problems would be very much appreciated. Cheers "Jack"