Additional piece of information on the possible cause

I have done a very quick assessment for the optical setups in the PSL room for ALS X PLL during a short stay in the PSL room today (17865).

Seemingly, the setup is not fragile against vibrations. It is unlikely that the mystery peak is something excited by external vibrations.

Assessment

Motivated by the past activity of checking out the mechanical vibrations (17263), I repeated similar tests. I gently knocked every optical component (e.g., a lens, HR mirror, corner-cube BS and such) relevant for the PLL by using a small allen key at a time to see if I can produce a vibration peak at around 250 Hz. The PLL X signal (ALS-X_PLL_SLOW_DAQ_OUT) was monitored in diaggui. Note that, in the previous test (17263), the error signals were monitored at ALS-X_BEAT_ERR_OUT, instead. This time, I switched the channel because SLOW_DAQ has a higher signal-to-noise ratio against ADC noise.

I have not seen a peak sticking out at around 250 Hz in any tapping tests. I was expecting to see a peak when tapping on the laser head as described in 17263. However, this action did not even give a peak at around 250 Hz. It rather gave elevated noise above 10 Hz all the way to a few kHz. This maybe an indication that some setup had been changed. Anyhow, it is gone.

Additionally, I noticed that the PLL Y setup was much vulnerable to vibrations. For example, tapping on the optical table with an allen key can produce appreciable elevation in the PLL Y signals (with a high peak at around 100 Hz) while the PLL X signals remained almost the same in its spectrum. This was true when I clapped my hands loudly -- Y showed noise elevation while X did not.

Current understanding

Here is an updated version of the candidate list (18442).

[Unlikely?] Mode hop of the Prometheus laser
[Unlikely?] Vibration mode of the laser head
Some other exotic origin
- RF interference
- electronic coupling
- Increase in laser frequency noise

If you come up with some other theory, please let me know.