I have a Powermax 155 boiler in my house. From new, and 10 years old this year.
Of late, it had started whistling extremely loudly just after firing up from cold, first thing in the morning.
It wouldn't do it after that first initial start up. Thing is, at 5am it got to the point where I wasn't setting the alarm clock anymore - the boiler and it's associated whistle would do it for me.
I'd hear the fan start as normal, then a metallic sounding 'click' and the burner would ignite, the the whistle would start. It would last around 10 minutes and then fade out, as if turning down the volume slowly on a stereo.
Not just a pain in the backside, this 'whistle' is as loud as a house alarm and can be heard from outside in the street, coming from the ridge vent. As the houses in this road are all the same age I began to notice 2 others in the same road who would also 'whistle' as loudly at different times of the day...
Strangely enough though, it only seemed to happen when the outside ambient air was very cold - i.e. the last week we've just had when it was down to -6 degrees C.
A member of British Gas' Home Care service, they came out and just from what I mentioned, diagnosed a faulty gas valve.
They replaced that, according to them it's a good £170 worth of part, and since then, fingers crossed, no more early morning wake ups. Although it's also not as cold outside in the mornings, so maybe time will tell.
According to British Gas, it's a common part to fail on these boilers and after 10 years, I've done well so far.
I hope this post helps anyone who is suffering from lack of sleep due to their boiler!
Of note, British Gas mentioned that despite being a decent enough 'lump' of a boiler, these things have a chequered past, due in part, to a fatallity in Oxford some years back.
Engineers were reluctant to work on them and even British Gas are unable to get technical combustion info from Baxi Potterton.