i hear the low hum... but only inside?

Lately I've been hearing a low frequency hum inside my apartment. I've searched all over, it's not an appliance. It's not coming from anything in my living space. I walk out into the hall and put my ear on peoples' doors, and it's not coming from their living space either. I step outside and I can't hear anything. I've experienced this before at my last apartment, but it turned out to be an idling motorcycle outside. This is different but similar. Any ideas? North of the Chicago area is the location.

it's coming from inside your head

Speakers from a stereo emitting that hum maybe?
Fluorescent mercury filled light bulbs?
Electricity in the walls?
Lost cell phone on vibrate buried in the couch cushions? I hate when that happens.

if anything i think its like the central air system of the apartments? i can't imagine what this could be, maybe pressure in the pipes? its just so bassy, some people can hear it some can't... thought it was tinnitus, airplanes, idling engines outside... im wondering if maybe theres something up with the atmosphere and the plane traffic is causing excessive bass frequencies... i dunno, nato was just here, so maybe something with drones? lol.. man im a nut huh.

Still hear the noise, it's not in my head. I recorded it.

soundcloud.com...

Any ideas? Could this be from water pipes vibrating due to too much pressure or something? I only hear it in my apartment.

Oh that's easy.... it's like this. Big brother is targeting you. What they're doing is they've got a satelite sending down radio active beams on your apartment. Usually they do this to try and cause cancer in the person living there so they can test out there weapon. that's why there's a buzz. Are you feeling any different?