Quoted from lordnorth:Randomly (but not often) I will hear a solenoid fire under the machine while playing, most likely from a vertical up kicker, maybe an opto is bad.
Funny, my machine does this too. It fires twice and that's it, very random. I'm assuming it's the upkicker to the gumball machine, but not sure. Haven't found it, but also haven't replaced any optos.
I tried to test all the solenoids and listen for the same sound but can't pick it out.