I measured sensor spectra of ITMY tower (fig1).
Strange bump can be seen in BF V2 LVDT at 10-20 Hz and IP ACC H1 at 2-5 Hz.
I checked these spectra during O4a (fig2) and BF LVDT V2 already have bump at 10-20 Hz but IP didn't have bump at 2-5 Hz.
I also check geophone signals just before and after vacuum venting (fig3), and the bump can be seen only after venting.
So, it is very likely due to the air inside the chamber and can be negligible after vacuum evacuation.