Wow, another awesome EVP. Especially the fact that you audibly heard it and reacted to it.
Do you have and video of this point in time?
That's pretty clear case of whats called "Infra-sound". Its a sound wave which is below vocal range.
And it was recorded loud and clear, and its obvious to hear that its not mechanically created.
Great catch, and also impeccable quality on the recording.
