On Nov 10th people did a test of turning on the ITMX bias, and saw no noise added in DARM. 45183  Robert and I did the same test Dec 6th and we did see that there was a noise peak at around 58 Hz, which came and went with an on off test and was pretty similar for positive and negative biases. 45720

I've gone back to the times of the Nov 10th test and made the first attached plot, which shows that there really wasn't a peak around 58Hz at that time.  

It would be interesting to know what might have changed with the ITM ESD or other electronics around the ITMs.