[Kenta, Yuzurihara]

We found the cause of the peak around 155 Hz. By turning off the OMC ASC beacon, the sensitivity was improved at the mountain around 155 Hz, sharp peak at 96 Hz and also 60~110Hz broadly. See the screenshot (Fig).

Bacoground

Today I recognized, by using Gauch, that when the LSC_LOCK guardian reached at the OBSERVATION state, the strange non-Gaussian noise was contaminated (Fig) and its frequency was reducing in time, from ~200Hz to ~150Hz. (Fig by Pastavi)
I chatted with Kenta-san about it. This phenomena started from 11 days ago. The timing was coincident with the work of the OMC ASC beacon and the implementation of the beacon excitation. We turned off the beacon excitation by changing value of the K1:SEN-ETMX_L_OUTF_GAIN from 1 to 0.
As a result, the sensitivity was improved around 155 Hz, sharp peak at 96 Hz, and the broad noise around 60~110Hz. See the screenshot (Fig). The brown and red curves are the sensitivity before and after turning off beacon excitation, respectively. (I checked quickly, so we might miss the improvement of the sensitivity in other frequency bands.)
We are not sure the contamination path from OMC beacon to DARM. It's necessary to investigate.
We tried to reproduce this phenomenna by turning on the OMC beacon. We observed the peak moved from 200 Hz to low frequency, slowly. The time scale to reach 155 Hz needs several minutes. See also (Fig).