I live in windsor ontario canada. I'm hearing the same thing. it sounds like a neighbor with a loud stereo and the bass is thumping periodically. I stand outside on the porch and I can't pick out the source of the sound... the entire neighbourhood is dead silent. Now we're right on a fault too on the us border. the st. lawrence river runs right into the great lakes. and that was another possibility i thought of. My cat doesn't seem to notice or he just doesn't care. It's been around for a while that I noticed. strange thing is i don't even really remember when it started. But I can always think of an instance that I remember the thump. not always a hum, but i do hear a thump.