We recently observed a periodic seismic noise in the CEB area affecting nearly all F0 suspension accelerometers.

Although this noise is effectively filtered by the SA chain and seems not visible in Hrec, its roughly three-hour cycle and characteristic double-pulse structure (two strikes separated by about 12 minutes, with minor timing variations) make it worth investigating.

Initial mapping with Newtonian sensors pointed to the northeast sector of CEB, and subsequent analysis narrowed the source region to the FCIM and FCEM zones.

Remarkably, each cycle unfolds as four pulses: an initial event in FCIM (or broadly the CEB area), a second in FCEM about 30 seconds later, then after roughly 12 minutes a third pulse again in FCEM, and finally a fourth back in FCIM/CEB about 30 seconds afterward. After three hours, the cycle repeats itself.

The implied propagation speed of ~10 m/s rules out a traveling‐wave origin.

Preliminary checks show that neither the (unlinked) HVAC units in FCIB and FCEB (per Davide Soldani, personal communication) nor the air compressor 150 m from CEB (which is under continuous monitoring) correlate with this noise, leaving the true source still unidentified.

It might be useful to place some seismic sensors near the HVAC units in FCIB and FCEB to shed light on this problem