Maybe you're in one of those places that experiences "The Hum".....

https://en.m.wikipedia.org/wiki/The_Hum

It's funny you should mention this.  There are some strange things happening in this area.
 
I was able to measure the loudest frequency at 57 Hz.  At higher frequencies the SPL decreases.  I'm wondering if it could be coming from the wiring in the house. 
 
I did some experimenting with high pass filters and Izotope RX3 noise reduction, and it can be filtered out pretty effectively, so I may be stuck with processing my tracks this way.

aspenleaf
I was able to measure the loudest frequency at 57 Hz.  At higher frequencies the SPL decreases.  I'm wondering if it could be coming from the wiring in the house. 

 
When it's below -30 outside my daughters bedroom turns into a speaker for the utilities transformer a couple of houses away.  The wire coming into the house is attached to her wall and acts like a string walkie-talkie.  Very loud and annoying hum that changes as you move around the room.  It's inaudible at higher temperatures, but problably still there to some extent.

Interesting.  There is a transformer a few hundred feet down the road.  The next time the power goes out (which is fairly frequently) I'll have to take some readings.

 
36 dBSPL may be near the lower limit of your meter's reliable range. They typically start to register their own internal noise below 30 to 40 dBSPL. Meters that work reliably down to virtual silence do exist, but they're very expensive. At any rate, even if the reading is accurate, 36 dB isn't terrible. 

That's probably the case with this meter.  I don't have one that's highly accurate.