Two noise sources recently appeared in External Magnetometers.

One is the usual SNAM Natural Gas pipeline Galvanic Current Protection System, it started at 02:15 UTC (circa) on Sunday 27 November.

The second one is a noise that started as irregular sidebans of the 50Hz mains, located at +/- 16.6 Hz, at 15:00 UTC (circa) on Monday 28 November, and moved soon to a broad line (a bump) located at +/- 27.7 Hz (as to say: 22.3 and 77.7 Hz).
This last noise seems originated inside Virgo, because it is louder as seen into the MC vacuum pipe ENV_CEB_MAG_MC_TUBE (0.4 nT), then following ENV_CEB_MAG_W  (0.08 nT), and last all others like MCB or EXT or NEB magnetometers (< 0.02 nT) - nothing is visible in ENV_WEB magnetometers.

We've looked at many INF channels, but haven't found any correlations yet.