I've been fooled by low sounds before. Most recently I thought the air conditioner outside my bedroom was erratically creating a low frequency hum that I could only hear when I sat in a certain spot near the window.

Well, it turned out to be the add-on hard drive for bedroom DVR, which was inside a wooden cabinet and drumming its normally inaudible spin vibration throughout the cabinet - and yet could only be heard if you were in a very small cone directly in front. Added a bit of padding between the two, and no more problem.