No clue for the mystery noise, but it's not the jitter from HPL linearly coupling to DARM.

In the attached, you see coherence between darm and usual things as well as DBB beam jitter signals measuring the HPL jitter. Nothing is obvious about mystery noise at e.g. 100Hz.

DBB PMC was locked, aligned using WFS, and WFS control was turned off. IFO was locked at nominal low noise with 40W.

We will gain something by retuning SRCL and MICH FF for f<100Hz, but these are not the main sources of the mystery noise at e.g. 100Hz. We might also benefit from pushing the CM gain to lower high kHz noise.

There's some coherence with ISS second loop sensors between 100 and 1000Hz. (Note that these ISS second loop signals are NOT what is plugged into the second loop board, they are digitally generated from indivisual PD output and that's how we still retain PD58 sum.) It's not clear if this is the intensity noise, changing ISS second loop gain might reveal something but this cannot be the mystery noise.

DBB_QPD_1DX etc. are WFS errors, and DBB_QPD_1QX etc. are WFS DC pointing. Apparently there's some jitter peak at 1kHz which shows up a bit in DARM, and there might be something at about 30-40Hz, but except these, nothing. PIT WFS signals (PSL-DBB_QPD_1DY and 2DY, these are blue and brown in the middle panel) might be showing the same broad 100-1000Hz thing that the ISS sees.

In the second attachment, I measured the IMC WFS and it shows some structures which we might have to do something about once we get rid of the mystery noise, but it doesn't look as if linear coupling of the jitter explains the mystery noise. (In this second attachment, strange bump in DARM at 300Hz is because something was injected for calibration measurement.)