Quoted from yancy:

Recently I was getting a second ball served and a few coils randomly firing. Turns out the coils were triggered by switches on the same matrix row as a trough switch. In switch test, I wiggled a bundle of wire leading to the trough switches: ding-ding-ding-ding-ding! Game sounded like a phone ringing. I re-soldered all the trough switches, problem went away.

Quoted from Pinballerchef:

You figure anything out on this one yet?? My METPRO is behaving in the same manner....every once in awhile it kicks out a second ball after launching the first one off of a skill shot try. It happened to me both times when I had selected BLACKENED as my song....doubt that has anything to do with it though.
Also not sure if this is normal but when I hold in the left flipper to get the skill shot....I get a buzzing noise that dies down. I know this is how EM's sound off flipper holds but the right flipper doesn't do this?! It's loud enough to be quite noticeable.