I suggested that if the vibrations of the HAM5-HAM6 septum plate were causing scattered light noise (note that Robert suspects not) one could try to damp the vibration mode with one of the SUS vibration absorbers. Dennis asked how effective this might be, and I estimate that it would be no more than slightly effective. 
I estimate that vibration mode of the septum plate is around 110 Hz (with a large margin of error)
I estimate that 1 VA might put a lower bound on the Q of 100-200.

more notes and some matlab calc can be seen in the "Secret SEI Log"
https://alog.ligo-la.caltech.edu/SEI/index.php?callRep=712
( your ligo.org credentials will let you in)