I turn the front windscreen heater switch on and although I can hear a slight hum there is no blast of fan and wind noise associated with the heater working. No air at all was discharging through the vents. I then tried the normal heating switch, dialing through the intensity of fan from 1 - 6 and, as with the windscreen function, there was no air or heat coming through any vents. The windscreen function was working well last week although I have no idea if the internal heat vents were. I have not used them since last Winter as it has not been cold enough. There is a slight hum as if something is straining to work when each function is turned on. HELP. Any ideas ?