As reported before, we see very often bumps in DARM spaced at 4.05 Hz.

I wrote a script to look at all 6000+ DQ'ed channels and look for peaks at 2 Hz and at 4 Hz. A peak is defined in this case as a spectral feature where the ratio of the ASD at 4 +- 0.3 Hz over the ASD at 4 Hz is at least 1/5.

The interesting result is that many PEM signals have a large peak at 4.05 Hz: mostly magnetometers and radio antennas, but also other PEM channels

This is the list of ALL Dq'ed channels that have a peak at 4.0x Hz:

           H1:PEM-CS_ACC_ISCT1_REFL_Y_DQ 4.060 Hz
            H1:PEM-CS_ADC_5_29_2K_OUT_DQ 4.060 Hz
         H1:PEM-CS_MAG_EBAY_LSCRACK_Z_DQ 4.060 Hz
         H1:PEM-CS_MAG_EBAY_SUSRACK_Y_DQ 4.060 Hz
          H1:PEM-CS_MAG_LVEA_VERTEX_Z_DQ 4.045 Hz
    H1:PEM-CS_RADIO_EBAY_NARROWBAND_1_DQ 4.060 Hz
    H1:PEM-CS_RADIO_EBAY_NARROWBAND_2_DQ 4.060 Hz
    H1:PEM-CS_RADIO_LVEA_NARROWBAND_1_DQ 4.060 Hz
    H1:PEM-CS_RADIO_LVEA_NARROWBAND_2_DQ 4.060 Hz
         H1:PEM-CS_TILT_LVEA_VERTEX_T_DQ 4.060 Hz
               H1:PEM-EX_ADC_0_09_OUT_DQ 4.040 Hz
          H1:PEM-EX_LOWFMIC_VEA_FLOOR_DQ 4.040 Hz
         H1:PEM-EX_MAG_EBAY_SUSRACK_Y_DQ 4.040 Hz
      H1:PEM-EX_TEMPERATURE_BSC9_ETMX_DQ 4.040 Hz
               H1:PEM-EY_ADC_0_14_OUT_DQ 4.045 Hz
         H1:PEM-EY_MAG_EBAY_SEIRACK_X_DQ 4.045 Hz
         H1:PEM-EY_MAG_EBAY_SEIRACK_Y_DQ 4.045 Hz
         H1:PEM-EY_MAG_EBAY_SEIRACK_Z_DQ 4.045 Hz
         H1:PEM-EY_MAG_EBAY_SUSRACK_X_DQ 4.045 Hz
         H1:PEM-EY_MAG_EBAY_SUSRACK_Y_DQ 4.045 Hz

Some of them have a narrow peak at 4.040 Hz, most have a narrow peak at 4.045 Hz, and a few have a broader peak at 4.06 Hz.

I could not find any change in the peak amplitudes in those channels when we see the bumps in DARM. This might suggest that 1) the peaks are a coincidence 2) there is some non-linear effect implied, for example if the radio antennas are measuring noise around some VCO frequency, and the VCO drift around and downconvert the noise from time to time.