Update.

The servo is a JR sport retract servo.
Connecting and re-connecting the servo leads seem to make it worse. (sorry Dan, I see this was your idea, I slept between posts.)
I moved the end point adjustment and sub trim to max values (both directions) with the gear down and chattering... nothing changed, no different sound. So I don't feel like it was a linkage issue.

Even with the chatter the gear is down and locked. What about interference?

Red