Just had a lockloss. It started with a long string of continual EY saturations, some broad "low frequency" noise (roughly 10-50 hz) appeared in the live DARM spectra, then a lump appear around 50 hz. This eventually became a sharp peak at about 47 hz that would get large for a few seconds, then settle down a little. Attached spectra show a measurement just before the lock loss (red) with the 47 hz feature, a measurement with the low frequency stuff before the 47hz peak came up (brown), and a measurement 2 hours ago (green). 47 hz sounds familiar, but I know not from whence.
I add: during hand off it was mentioned the low frequency stuff had been poking up all day occasionally. And this didn't get bad until after everyone else went home, but looking at the verbal log, it looks like EY saturations had been increasing in frequency over this lock stretch, but not in a noticeable way.
Looks like it might have been one of the 4.7khz violin modes. Red is from right before a lockloss just a couple minutes ago, green is from the earlier lockloss, blue is from around the time Jeff had damped down this mode. Looks like the lower frequency mode got rung up somehow and is still pretty high.
