This line (re)appeared in Hrec (and DARM)  on Dec 21Hz: Figure 1. and it is still there pretty loud.

The line is present in accelerometers monitoring CEB  tower tanks vibrations and it is loud in the CEB microphone. Coherence with Hrec is large for example with ENV_DT_ACC_Z (Figure 2).

Since the amplitude of the line in acc and mics did not change before and after Dec21, we can suspect a change occurred in the coupling to DARM.

Looking at the logbook for documented actions:

• Dec 19 afternoon (44167, "OMC mode matching shift - wednesday"). Yet after this work the ITF relocked on Dec 20 morning (44163, "Flat noise and differential offset(s)") and the peak is just barely visible (Figure 1)
• Dec 19 afternoon and 20 Dec afternoon  (44041, "actions related to problem with WE_MAR_Z_CORR balancing")
• Dec 20 afternoon - Dec 21 morning (44168, "Sync problem investigation and recovery")

but it is not clear if and how any of these actions could have increased the coupling.

This line was detected also in the past, for example in August. A suspected source are the big turbo pumps at the BS links.