We observed peaks around 4Hz on spectra taken last week on HAM3-ISI. LLO folks suggested it may come from the purge air.
After talking with John and Kyle, we agreed on me turning it all the way down for about an hour. I will now take spectra in this configuration and put the butterfly valve back to the position I found it in (fully open) once I am done.
I just put the purge air back into its initial position: fully open.