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.