Furman Elite 20 PFi - Hummmmm

A while back I purchased a used Furman Elite 20 PFi for a questionably low price that represented little risk. The unit emits a steady audible, but not distracting for music, hum. I have opened it up and tried to determine the source of the noise, but have not had much luck. Slightly concerning was that the unit had clearly been opened before. There were cut zip ties laying about, and after some research it appears these were from the chokes, which were now unencumbered. I re-secured these with two ties apiece, although I now see an online photo showing four ties on each.

Anyway, the sound is not a distraction like I said, but I am curious if it is adding noise to my lines or represents a safety concern. If it matters, I have a dedicated line, and I believe relatively clean power.