Figure 1 shows that during the time when PMC only was locked (IMC unlocked) the increase in noise in the PMC channels corresponded also to an increase in noise in the master laser DC monitor (but not in the frequency monitors). 

Figure 2 and 3 if you compare the normalized spectrogram of the PMC error signal and of the master laser DC monitor, the increase in noise happen at the same times. So clearly they have a common origin.

This raises two questions:

• What is exactly the sensor for the PSL_ML_DC_MONIT_FS channel, and where is that sensor located in the optical path?
• Is there any feedback applied to the master laser in this configuration (PMC locked, IMC locked)?

If there is no feedback applied, then it points to the master laser as the origin of the problem (or whatever that is between the master laser and the ML DC monitor). Or that the electronics that are providing the feedback are creating their own time varying noise, which could be tested by physically unplugging the cables from the master laser.