Low "Hum" Frequency Detector
Low frequencies and directionality don't mix very well:

From http://en.wikipedia.org/wiki/Sound_localization#Evaluation_for_low_frequencies


  For frequencies below 800 Hz, the dimensions of the head (ear distance 21.5 cm, corresponding to an interaural time delay of 625 µs), are smaller than the half wavelength of the sound waves. So the auditory system can determine phase delays between both ears without confusion. Interaural level differences are very low in this frequency range, especially below about 200 Hz, so a precise evaluation of the input direction is nearly impossible on the basis of level differences alone. As the frequency drops below 80 Hz it becomes difficult or impossible to use either time difference or level difference to determine a sound's lateral source, because the phase difference between the ears becomes too small for a directional evaluation.


Now here is what I would do (I'll be making a few assumptions). Rather than building a intricate device to determine where the vibrations are coming from do the following when you hear the vibrations:


Go to your home's electrical distribution box
Switch off all power to your home
Go back to where you heard the hum


If the noise is gone you now know it is electrical in origin (just to be sure). Even though you said you switched off everything you did not say exactly how so make sure it is EVERYTHING by doing the above. If you still hear it, go outside, walk around, where do you hear it? By narrowing the parameters down of where you hear the noise will help you to easier locate the source.

If this does not help I would try to use one of these android applications:
https://market.android.com/details?id=radonsoft.net.rta&amp;hl=en

https://market.android.com/details?id=com.DanielBach.FrequenSee&amp;feature=also_installed

You should be able to follow the low frequencies depending on the quality and sensitivity of the microphone inside the device. I suggest testing this with your PC first before trying it on the real problem: http://onlinetonegenerator.com/

Here are some devices I have found to cause low frequency vibrations:


Refrigirators
CRT Screens
Florescent lightbulbs
Pumps
Day-night switches on streetlights
Water flowing through an loosehanging pipe (yes, believe it or not)
Computers (duh)


Best of luck finding that buzz!