[Jenne, Hang, Gabriele]

This morning, when we were locked at 2W, we saw that a line at 28.45 Hz was ringing up, and visible mostly in MICH / PRCL and SRCL, and a bit less in DARM. After checking all loops for possible instabilities, we decided it was a mode of some suspension. We added band-stop filters at 28.4 Hz in all angular degrees of freedom of all suspensions, and this solved the problem, letting the mode ring down. We haven't identified which suspension is responsible.

The mode (actually it looks like a doublet) was also visible in the two ITMs optical levers, but at that time we saw the same excitation in the hard angular loops.