Changed it to three sensors and it still does it.
If I unplug either the trigger or echo pin, the chatter stops.
If I point the sensor at a solid object 10 cm away, the chatter stops. If I point it off in the distance, the chatter resumes. It's not really bad chatter, just a degree of movement, but it shouldn't be there for a servo not under load.
There is no serial read event occurring unless I move a servo, so I knows its not that.