Our Carver/blown air heating system is not emitting any heat on the electric setting, but the fan is working. On the gas setting whilst it is reaching temperature it is emitting a loud humming noise/vibration is this normal? The fan etc is working and emitting loads of heat but just rather noisey. If this is not normal has any one any ideas as to what these problems might be? The caravn is a sterling sheraton 1999. Thanks.