Why does a Rangemaster 100 go bang? ?

The cooker is electric ovens and plate warmer and gas hob. When not being used with only gas and mains on there is at random times an explosive noise. Sometimes it sounds like a pressure release at others it is much more explosive. It is not gas, turning off the mains stops it.
Any ideas as to the cause? Is there a large capacitor in the electric circuit that could be breaking down ?
How do I get into the control panel ? The bottom screws are easy but the top ones need the removal of the top plates. How is that done ?

CB of EK
April 2014