Over the summer, Sam, Robert, and I investigated several peaks in DARM, including one at ~35.4 Hz and a peak that oscillates between 30-40 Hz. The 35.4 Hz peak was found to couple strongly at HAM1 and HAM6 (see alog 79546). Shut-offs of the office area air handler and the main chillers showed that the peak decreases when these systems are shut off, though it doesn't disappear completely. See Figure 1 and Figure 2 with office area air handler shut offs at 19:33:00-19:38:30 and 19:43:00-19:55:00 (alog 79888), and Figure 3 for main chiller shut offs (alog 73941). Note that there is also a fainter peak about 0.2 Hz above the 35.4 Hz peak as seen in Figure 4 but it is unclear if it also decreases when the HVAC is shut off.

The oscillating 30-40 Hz peak tends to be near 40 Hz at the beginning and end of the UTC day and drops to near 30 Hz in the middle of the day as seen in Figure 5. It often appears sinusoidal as seen in Figure 6 but sometimes has a more random appearance as in Figure 7. It is seen in microphones and magnetometers as well, and first appeared at least as early as 6 June 2024. Its source has yet to be identified.

These peaks were dicussed in a presentation at the end of our summer fellowship as well as in a DetChar call, now posted on the DCC: LIGO-G2402140.