noisy myson midas b combi boiler

[Charger]

got a noisy myson midas b combi boiler, only when heating hot water, it’s quiet as a mouse when central heating is on, hot water is hot, central heating radiators are hot, no concerns there

with the midas b you can choose to preheat a small amount of hot water so that you get hot water quicker when you turn on a tap. so from cold, when I turn the selector to ‘instant hw’ (or turn on a tap) the boiler fires and the hw pump starts, at this point all is quiet. after a minute or so as things warm up, the system starts to emit a droning noise, this gets louder and louder, until the water gets up to temp. then as normal, the burner cuts out, the hw stops, the ch pump starts to clear excess hot water from the heat exchanger, and as soon as the ch pump starts the noise immediately stops

I tried repeating the above from cold, but I turned off the gas valve as the droning noise built up, immediately the noise stopped when the burner went out but the hw pump carried on running, so I don’t think it’s the hw pump

I’ve bled all radiators and vent points on the boiler. also tried switching hw pump from speed 2 to 3, made a slight but very little difference

any ideas??

many thanks in advance

 

[ChrisR]

3 options - all quite reasonable at this age
1) get a corgi to check/adjust the gas valve
2) put up with it
3) get a new boiler.

 

[Charger]

thanks for the rapid reply

1) can I presume that you’re suggesting that the gas pressure may be too high and the boiler is over heating??
2) that’s a possibility, just gotta make sure we don’t run off hot water at unsociable hours so it doesn’t disturb the neighbours!!
3) also a possibility, to my knowledge the boiler is 18 years old