Have you tried putting your ear against the cabinet, about one third of the way done from the top on the left-hand door of a two door cabinet? Depending upon ambient noise, there is usually some indication that the electronics are "powered up" even if the fans are not currently turning. The fans produce a mechanical noise / swish, the electronics (more succinctly the mains transformer) can hum.
Having said that, I do agree with
Ribble's comment. You could be experiencing a very quiet pair of cabinets.
-----------------------------------------------------
100% Linux and, previously, Unix.