I seem to agree with Lucia.

[Note]
= ETMX =
First, I was hpoing this might caused by coupling from vertical motion (since the GAS-keystones and the BF were oscillating with amplitude of ~10um(/~10urad) in RMS, in ETMX) and thus I impelemted 2 damping filters at F1 and BF-GAS of ETMX(Not much tuned). Those vertical motions were damped in some extence though, the issued peaks were not dissapeared, i.e., coupling from vertical was not the cause.
Before/After: