I have went to a customers house twice this week. They are complaining about there vokera unica 32HE making a loud humming noise during the night and waking them. I have not witnessed this the two times i have been on site and they said it doesnt happen during the day when they have the boiler running. I checked that it wasn't on the pre-heat water mode, which it was not and also adjusted the fan speed down to see if that would improve the problem, it has not. I have bleed all the radiators making sure there was no air in them. Any advice would be great...