This morning I searched for the sources of these lines.  The two fans serving clean air flux to the SPRB minitower area seems the source. It is advisable to perform a switch off  once SPRB is in vacuum. Other tests done with CEB clean Rooms air conditioning, and with Antonio's fan cooling SR turbo pump.

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Using the accelerometer of TCS NI bench (the one which showed the highest coherence with DARM) I tracked down the two lines which today are at 16.7Hz and 16.9Hz (Figure 1, top-left).

With Davide we first switched off the CEB Clean Rooms air conditioning between 9:07 and 9:012 LT: this affected Lines at 17.9Hz and 18.5Hz which disappered from many accelerometers (Figures 1 and 2) but not the hunted lines.

Lines which appear to correspond to the searched ones are emitted by the two fans serving clean air flux to the SPRB minitower area. Figure 3: picture.

• Figure 4: accelerometer ENV_SPRB_LINK_ACC_Z was temporarely attached to the chassis of one of the two fans (the one on the right when looking east), this emits a vibration around 16.7Hz
• Figure 5: the same accelerometer was temporarely attached to the chassis of the second fan, this emits a vibration around 16.9Hz.

I also checked the fan which is being used to cool down the SR turbo (I used channel ENV_SR_ACC_Z temporarely connecting it  to a meggit accelerometer attached to the fan' base. See picture in Figure 6). It emits vibration lines at 4.5Hz and around 31.Hz plus several harmonics above 50Hz, which however presently do not show evident coherence with accelerometers or microphones: Figure 6.