End of Stroke switch is dirty, worn or needs adjustment.

With the game off, manually move the the good flipper and observe the gap in the switch - as in "when" it opens and adjust the faulty EOS to open at the same time or travel.

Yep..EOS is misadjusted. These actually need a bit of tension on them to work properly so make sure the short arm is pushing well against the long... but obviously not so much that the switch doesn't open properly which in itself can lead to "chatter"