@cdc What are you using to verify that it was 40hz? Do you have an HVAC system near the room? As silly it my seems, I have a 30hz peak in my room and it is similar in size so every time the AC unit is on which is right above the room, presents rumble at that frequency. I use a RTA to measure room response every now and then when I change out speakers or subs for that matter.
Another situation is consistent of balance of frequency response of a bass guitar or any matter of instruments that has frequency response that low.