Followng Antonio's request we had a look to possible magnetic emissions from the electric heater being used to prevent water condensation on the  LN2 pipe of the NE cryotrap.

We found some  1Hz-spaced noisy lines in NE magnetometers and UPS voltage sniffer whose appearence is correlated with the operation of the heater: see Figure 1.  Signal MB_LN2OUTER-HEATER measures the percent duty cycle of the heater controller. As also evidenced in figure 2, the noise is absent when the duty cycle is either zero o 100 percent.

This resembles what observed for the MC building heating. Thus, we suspect the observed noise is the effect of some PWM controller which switches on/off with a constant repetition rate of 1Hz and a variable duty cycle, causng a modulation of the UPS voltage where the device is plugged in.  Our interpretation is that when duty cycle is 0% or 100% the switch keeps always open or closed and modulation stops.

The issue is to be further investigated.