I added aline at 20 Hz on MC2_M3_L, amplitude 1000 counts. The line is very well visible in the IMC transmitted power. I used a modified version of my phase tuning script (attached) to plot a time series of the transfer function PSL-ISS_SECONDLOOP_SUM14_REL_OUT_DQ over SUS-MC2_M3_LOCK_L_EXC, which should be zero when the IMC is locked on top of the resonace.

However, as visible in the attached plot (both real and imaginary part of the TF over time, it's ot possible to zero the transfer function, but basically only to rotate the phase. My guess is that the longitudinal actuation has a too large coupling to angles, that in turn generate intensity noise. We should repeat this measurement with a frequency line.