We have identified the source of a comb with a fundamental frequency of 11.111 Hz that appears in the DARM channel and also in the PEM magnetometer channels. 
 
The noise source appears to be a particular pressure sensor with the frequency given by the communication with the Beckhoff computer. We measured the magnetic spectrum at BPG402-SE pressure sensors both in the EY station and in the EE shop, and we saw the 11.111 Hz comb that we were looking for (see the attached plot for a spectrum). The frequency measured in the EE shop was either 11.111, when there was one PLC task cycle time of 10 ms, or 11.9 when Patrick set up two tasks with settings of 1 and 10 ms. Using a strobe light, we found that the LED on the pressure sensor also blinked at 11.1 or 11.9 Hz.  As expected from the settings, the Beckhoffs in most of the racks at EY produced an 11.9 Hz signal (detected by magnetometers on the slow controls boxes, but not seen in DARM), while the one in the vacuum rack produced the 11.111 Hz signal.
 
The coupling to DARM is likely happening because the pressure sensor used in the ESD pressure interlock is sharing the power supply with the electrostatic drive. The 24 V power cable to the ESD was a strong source of 11.111 Hz magnetic fields.
 
The solution to this problem may be changing the power supply of the pressure sensor. This work will be done this week, and we will be able to test if the comb goes away when the detector is back and we can get spectra of the DARM channel.
 
Pep Covas, Robert Schofield, Patrick Thomas, Richard McCarthy