On Tuesday, there was a swap from Fan 1 to Fan 2 at End-X (alog 69635) in attempt to reduce the vibration levels at EX, and in turn prevent 4.1. Hz noise from appearing. Looking for periods of 4.1 Hz noise after this swap, there appears to be few, if any, cases of 4.1. Hz noise in the last 2 days. An example daily spectrogram is attached showing the 4.1 Hz noise before the swap. I've also included a daily spectrogram from today, which shows one time period that may have weak 4.1. Hz noise still present. 

More data is still needed to understand the full effect of this swap, but preliminary checks suggest that even if the 4.1. Hz isn't gone completely, the rate is lower compared to previous days in ER15.