One week ago (Monday April 20th) at 6:50 UTC (8:50 LT) a magnetic comb with 1Hz spacing turned on, and it has been on since then. The origin is still unknown. The noise is mostly affecting up to few tens of Hz, and has the characteristics of a clock driven noise, yet it is somehow intense and widespread. Here is what we observed so far:

• the noise was first noticed in the external magnetometer (thanks Renato)
• the source is most likely DC-powered since the noise is not consistent with 50 Hz sidebands
• comb lines are very narrow, with at least ms stability, and this points to some clock driven device.
• the noise is louder (a factor 10 to 100 louder than EXT) in CEB magnetometers and in magnetometers close to towers. NI, WI, BS are the loudest.
• the noise is present also in  the NE and WE magnetometers (curious!) where it started at the same time as for CEB
• no evidence of noise is seen in any VOLT, CURR sensor (UPS or IPS), as well as nothing in the grounding monitor
• the ELECTRIC monitor seems to show a factor two increase in the amplitude of lines at 1,2,3,4 Hz.
• Large coils are all off: VPM processes tell that all coils relay are set to 0, and in addition CURR and VOLT coils monitors do not see any noise
• the logbook does not report any concurrent operation
• VAC team and Davide are not aware of any new hardware or operation at that time
• the VPM log does not report any action at that time