I estimated the frequency-converted noise of the OMC Error -> Trans.
This calculation assumes that this noise amplitude is linear and the response function model is aberable, except for the harmonics.

I used the data of the single-line shaker injections (79, 80, 81, 82, 83, 84, 91, 92, 93 Hz) for the beam duct, performed on July 19. (klog30473)

Fig.1 (top) is the amplitude spectral densities of the OMC Error signals for each injection (colored) and the reference (back).
Fig.1 (middle) is the amplitude spectral densities of the OMC Trans DCPD for each injection (colored), the reference (back), and the projection of the Error->Trans (red). 
Fig.1 (bottom) is the response functions for each injected frequency.

The projected noise spectrum is 1-2 orders smaller than the reference data of the Trans DCPD.

By the way, the response function value corresponding to +60Hz looks to be about 4.5e-3 mW/count, for all injections commonly. 
In Fig.2, I added its projection by assuming this factor is common not only for the injected but also for all frequencies of the OMC Error signal (yellow)
This result is larger than the projection of the broad-band frequency-converted noise.
So the first work of noise hunting may be the 60Hz electrical coupling reduction.