I live in Calgary, AB and used to hear something similar to the Hum approximately 4 years ago. At the time I was in a basement suite and would occasionally wake up and would hear a humming noise (unsure if I was waking due to the hum or not.) Eventually, the conclusion that I came to was that because I was "underground" the low frequency of a train yard not far from my house was the cause of the noise.

My wife frequently tells me that I have sensitive ears. And I've had to leave rooms because of feedback in a sound system that no one seems to be able to hear.