Ambient noise levels

I'm trying to get the ambient noise levels in my studio down as far as possible.  I can't seem to get below 36 db (A weighted), even with everything in the house off, including the refrigerator.  My studio is acoustically treated with bass traps and mineral wool 2' x 4' panels, textured drywall and some diffusers on the ceiling and walls, and hardwood floors.  The ceiling is 9' tall and the room is about 12' x 16' in an irregular shape.  I have a few large baffles that I can move around where needed and there is an upholstered couch and a few chairs in the room, along with an assortment of instruments and amps, including an acoustic drum kit.  I don't get any strange reflections or flutter echos, and I cover the drums with a blanket to keep them from resonating when I'm not recording them.  I checked ambient noise levels in other rooms in the house and they are around 42 db.  I live out in the country, and the home is built into a hillside, with two of the walls of my studio being concrete, insulated with styrofoam sheet and covered with drywall.  The other two walls are standard 2x4 construction with drywall on both sides.  I don't usually have outside noises making their way into the studio.  Is it unrealistic to expect noise levels lower than what I'm getting?  I was hoping to get down to around 25 db.  Any advice?

From a practical perspective is the ambient level consistent enough to be removed by a noise gate or noise reduction tool? It may be unrealistic to expect super low levels, and the gate approach helps to deal with "what is" and be usable.

A lesson learned for me is low levels led to mic gains set too high, so transients like body motion were picked up. Consistent background noise is easier to accommodate than transients getting through - in your case a simple gate may be an easier path for your recording needs. 36dB is not excessive if intended material is substantially louder (much comes down to S/N ratio in the end).

There are no windows in the room.  Measurements were made with nothing turned on in the room.  No computer, no rack.  I even turned off the clock, lights and surge suppressors.  The refrigerator is on the floor above this room, but about 20 feet away.  I thought it might be making the floor resonate, so I unplugged it, but the 36 db reading remained the same.  I am measuring with a SPL meter, but even just using my ears, it seems there is a background noise, but I can't seem to locate it.  It seems to be throughout the house.  When recording most things, the signal to noise ratio is fine, but I first noticed the background noise when I had a ribbon mic going through a Presonus RC500 preamp and recording spoken word.    But the ambient noise I'm referring to is not electronic noise from the preamp gain being turned up because of the ribbon mic.  There was some preamp hiss above about 50 db gain, but I started trying to track down the noise, and was surprised that with everything shut off that I was still getting 36 db of noise as read by the SPL meter.

Interesting thought, mettelus.  I live in the mountains in Colorado very near the continental divide at 8750 foot elevation, but the house is built on bedrock and now you have me wondering if that might be transmitting sound.  I'm very far from any industrial facilities, but I do have neighboring homes within a few hundred feet.

Another thing came to mind with this. Vibration transmitted into the air will be less than the surface creating it (and vice versa). If holding the SPL meter is 36dB in "free air," try holding it firmly against a stud on those walls (if is easy to find one) and see if it creates a change in the level. Physical contact with offending surfaces should give higher readings (similar to a stethoscope).
 
If the SPL meter does not increase, those walls may not be the source but simple speculation.

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

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

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.