Prompted by me noticing on-off behaviors in the daily strain spectrogram for today at around 20.2 Hz, I've done some additional investigations into the source and behavior of this line: 

The 20.2 Hz line, which is currently prominent in DARM, first appeared in accelerometer and microphone data from the corner station on June 9. The first appearance of this line that I found was in the PSL mics, as shown in this spectrogram. This line then appeared in DARM in the first post-vent locks a few days later. The summary of work from June 9 does not show anything obvious to me that would be the source of this new noise.

This feature also turns off and on multiple times during the day. An example from today can be seen in this spectrogram. Most corner station microphones and accelerometers exhibit this feature, but it is most pronounced visually in the PSL microphone spectrograms. I was unable to identify any other non-PEM channels that showed the same on-off behavior, but this does reveal many change points that should aid in tracking down the source. Almost every day, this line exhibits abrupt on-off features at different times of the day and for varying durations. Based on my initial review, these change points appear to be more likely during the local daytime (although not at any specific time).  When the line first appeared, it was usually in the "off" state and then turned on for short periods. However, this has slowly changed, so that now the line is generally in the "on" state and turns off for brief periods.