I pulled up a hertz generator, and as I hear it, was trying to match the frequency, it's goes back and forth from the mid-low 20s and up into the mid-high 30s. The pattern is 1 of two I keep hearing it switch:

1. ~36hz for 5 seconds > 24hz for 1 second > ~36hz again but for 10 seconds, then it drops to the other for 1, then back to the 5 seconds.
   
2. ~36hz for like 15seconds, then the lower one for 2 or 3, then the higher tone for like 20 seconds, backto the lower one again for 1.

I can hear it change clearly, and it is faint enough my kids couldn't hear it. I thought It was me, but I hear it in bed clearly, but when I go to another part of the house it's more faint or gone.

Right now for example, it's just on the higher tone, and it will stop for .5s and resume again.

Am I nuts? What could this be?