The optos on Stern SAM machines are notoriously unreliable and prone to chatter. Most often, the problem is that the little two-pin connectors on the transceivers don't make proper contact with the pins on the opto board. Re-seating the connectors fixes the problem briefly, but it usually comes back in short order. You can get get the same issue with the main connector on the opto board. You get chatter until you re-seat that connector, then the problem comes back again after a few days or weeks.

The connectors under the playfield are most likely to cause the problem because that's where you have the most vibration. Any connectors that are part of the switch matrix in the backbox can cause this too, but are less likely to do this.

Reseating all the connectors is usually the first step. Do this one at a time, so you can figure out which connector is the culprit. The switch matrix test can be useful identifying which opto is responsible. Then do something near the chattering connector, such as cleaning contacts, adding contact grease, re-flowing solder on the pins, etc.

I've seen people put a small dab of silicone on the junction between the connector and the PCB, to prevent movement. If that doesn't help, you can cut the connector off the ends of the transceiver leads and solder the leads directly to the pins on the opto board. If you want to be able to remove a transceiver without soldering, you can add an in-line connector to the leads. That allows you to pick a more reliable connector and, because the connector is suspended in mid-air by the lead, it tends to experience less vibration.