Following a question from Ben, I looked at ARS pedestals from the previous experiment (2004). Boards are different now, but daughter boards (mezzanines) are the same. It seems that we already had high frequency fluctuations, perhaps slightly smaller in amplitude (I plotted the figure at the same scale as the previous entry). I do not know if this pedestal corresponds to the same channel (I would need to do some research to find out the cabling matching on the previous experiment).
Fig 1: Typical pedestal from old boards (2004 experiment)