We moved into our home about 20 months ago. Not long after we started noticing a noise. Until today, my wife and I were apparently the only ones who could hear this noise. That's because it's nearly inaudible unless you stand in one place (the master bath toilet area) when the house is completely silent. The furnace must be off, you have to stand perfectly still, and nobody can make any noise at all.

Under those conditions, you can hear what sounds like a cat crying. It's a sound that rises and falls and then is silent before rising and falling again. It repeats this in a perfect cycle and never stops. As far as we can tell it has made this sound 24/7 since we first noticed it about 18 months ago.

Up until today I had no way of conveying what this sound was like, but a friend of mine has a much nicer recording device and was kind enough to help us try to capture it. He did his best and we've now ended up with this audio file. Unfortunately it's still VERY noisy, and there's a bad hiss. I tried removing it in Audacity, but it made the file very garbled. I hope you can hear it over the hissing. The sound happens three times in that clip, about every four seconds at the 3, 8, and 12 second marks.

Does anyone have an idea of what this might be? Can you hear it?