seeburg L100 popping while mech. is changing directions

hi All,
I have an L100 that has just been rebuilt. It sounds good, but it pops
in the speakers each time the mech changes direction. Is that the mech
mounted preamp picking up the contacts arcing? Mute is working because
when no record is playing, I can rub the stylus and there is no sound.
I was told by the rebuilder and have read that the 1957 Seeburg jukes
with the brds (instead of the hard wiring) tend to do this, why? How do
I correct this? Thank you, Rick.