The DARM noise between 18 an 21 Hz is due to calibration line upconversion by ASC motion. 
This is shown by the bicoherence between DARM and ASC-DHARD_Y, and also by the fact thta DARM is coherent with calibration line signals multiplied by ASC signals (see two examples attached)

We should repeat a test with CAL lines off again now that the low frequency sensitivity is better.