My Vokera Excell 80E combi boiler occasionally starts whistling when
the central heating is on. I can stop the whistling instantly by
turning on a hot tap for a couple of seconds. The whistling goes away
but always comes back at some point. Some days it will come back in a
few hours, other days it won't whistle much at all.

Please can you tell me what the problem might be and also what to do
to fix it.