[Former Proud Canadian]

It sounds like a loaded dump truck rolling down the street about six blocks away. A low hum like noise you would expect from a not so distant diesel engine or other machine.

I discounted it for months until I finally heard it a few weeks ago.

[Jack Hydrazine]

I would think that finding the source of the sound would be a great project for the nearest university with decent geology/earthquake department. They would certainly have the equipment sensitive enough to pick up on the vibrations and localize them to a particular area. Has anything been happening on that front?