Fix stationary fan on Vokera Excell 80sp?

Fan has always been running even when no demand for HW or CH. Boiler fires up ok for both & sounds like the fan runs, but when they go off fan is stationary. Can hear faint singing like a kettle & temp gauge rises. Turning on HW tap seems to reduce this. Pilot stays lit. Had someone look at it but they said fan doesn't need to run all the time. I don't believe them!