I spoke to someone today that has many years of experience in this area , many of things I will tell you that he said have been stated but he said ,
1)medal to medal
2)EI module not grounded properly
3)EI battery not insulated
4)Engine switch not properly installed or not buffered enought with some sort of anti vibration gromet
5)medal servo screws hitting medal servo keepers that run thru the rubber grumet
I know a lot of this sounds like overkill but I aked him for every known cause , basiclly and simply put he said medal to medal is the most common and in the least expected places
6)engine standoffs