If you don't hear a hum, don't look for it. Once you're aware of it, it can drive you nuts. I spent quite a while looking for the source of a low-frequency hum at my old house and never came up with anything. As far as I could tell, it wasn't just the sound of blood rushing in my veins or tinnitus.

Out of curiosity, is there a good sensor for picking up acoustical vibrations between, say, 0.5-50Hz? Would a MEMS sensor go that low and have a flat response?