The ~16Hz bounce mode of some optic is showing up in the Yarm error signal.
MC is kind of 'windy' looking, so maybe it's from that? (Yuta's guess).
We need to make sure that the SUS damping and oplev paths both have notches at the correct bounce mode, not the old, old MOS frequency. If that doesn't work, may need to put a resgain in Yarm path.