I have a Vokera Excell 80SP the pilot light is on however it is not firing up for either Hot water or Heating I have taken the front cover off and there is a ( can only descibe a humming noise) that has replaced the normal running sound of the heater before . If i press on the internal metal cover the noise defiates a little and then returns back to humming noise seems like a vibration defiation in its sound, has anyone any ideas on how i can rectify thankyou.