One of the performed tests consisted in closing the air valves of the inlet and outlet ducts while the HVAC was off. Acoustic modes of the ducts themself,  associated to air pressure waves resonating inside the ducts, would be affected changing their frequency or reducing. A similar test has been done for DET: https://logbook.virgo-gw.eu/virgo/?r=69058

The plots compare microphones inside the inj lab (and the horizontal accelerometer on LB) at two times with HVAC off: ducts open (blue) and ducts closed (purple). No significant change is observed.

The purple curve shows extra noise due to the fact that on May 19 the Tower HVAC was on (NI open). The blue line in Figures 2 and 3 was taken on April 14 2025 when the ambient noise outside the lab was much quieter (also the CR HVAC responsible a 18.5 Hz line was turned off at that time). 

Figure 3 zooms in the bumps region. Acoustic bumps peaks at 12.07 Hz and 18.20 Hz. The seismic bump present in the laser bench peaks at 18.06 Hz.

In 2021we did tests of mechanically stimulating the LB and measured a mechanical mode of the bench around 17Hz: https://logbook.virgo-gw.eu/virgo/?r=50496. This seems not present now... did it move to 18.06? Could be tested by tapping the bench.