Radiant Combination Boiler Whistle?

[smptrds]

Sometimes on start-up and often when hot water is turned on at the same time as the heating, the boiler whistles for approx. 1 minute.
If it starts on start-up, I can turn off and back on when the whistle stops.
Any good suggestions will be welcome please.

[The Builder]

Whistling; whooshing sounds; kettling (sounds like a kettle boiling) are just some of the noises in a boiler's repertoire.

Kettling can come from a slightly bunged-up heat exchanger. Cleaning or replacement needed usually.

Whistling can come from the diverter valve (directs water to either heating or hot water as needed). It often stems from a low flow rate. Maybe low mains pressure or, more likely, not enough rads open , or restriction in the pipework. When a hot tap is opened, the heating flow drops, making the flow to the rads worse.

Fitting an automatic bypass often helps when rad flow is reduced for some reason. This sort of returns the water to the boiler instead of forcing it through the system and whistling.

The manufacturer's should be able to point you in the right direction. Or, it would be helpful to have an experienced boilerguy have a listen to identify the sound.

So many possibilities as always with boilers. I'm sorry I can't give you a fix, but an experienced ear should be able to.

[Sycamore3House]

A Plumber was called to my elderly parents house some years ago with the same complaint. He told them that the only problem was that 'The boiler was happy and it was sounding happy'.
He did not charge (Thank Goodness) but at least 10 years later the boiler has now been replaced.

[smptrds]

Thank you both for your replies, I have my regular boiler man calling for a listen.
It has been identified by others as the 'O' ring in the three way valve.
Pressure o.k. New diverter valve fitted some 18 months ago.
Will let you know the outcome.
Thanks again.
Dave