As already noted in noise around the VELA, some hints, these peaks seems a modulation of the 0.2Hz noise in the dark fringe signal low frequency part and an "hypothetical" line at 23Hz (see the above mentioned eLog 29458).
We do not know what is special about 23Hz, however Paolo knows that the 0.2Hz is one mechanical mode of all the long SA. This mode is well visible in the F7 Lvdt signals along x and z. Thus, following Paolo's suggestion, I performed a similar exercise as in eLog 29458, and cheched the coherence between dark fringe and signals Sa_xx_F7_x(z)Lvdt modulated at 23.000Hz. The coherence is significantly larger with the NE xLvdt (Figures 3 and 4). Figure 2 shows the Sa_xx_F7_x(z)Lvdt signals (I do not apply any filtering to these signals) and Figure 3 shows the coherence. Figure 4 shows the same coherence zoomed-in around the VELA.
We do not know what is "magic" about 23Hz, but If I move just 0.01Hz away (thus modulate at 23.001Hz) all coherence disappear: see Figure 5.
I let Paolo to eventually comment on the meaning of this result.
/virgoData/PrCali/PermanentLines.cfg:
# nameOut - Amplitude - sinus Frequency - phi0 - offset - Unit - fType - sampleRate - rampTime
PR_SINEWAVE_CH Ca_NE_zMar_l1 2e-5 13.6 0. 0. "au" normal 10000 5
NE bottom-stage DSP:
COMB tyPSDm 9_4Hz.cmb no 3 lin_tyAA
Since the noise is clearly caused by some sort of up-conversion, some noise could be expected around the sum-frequency of 13.6+9.4 Hz. Note that zMar and ty are applied to the same two coils of the marionetta. It would be interesting to switch off these lines or move them in frequency at a moment the bumps around 23 Hz are visible.