I found that the shielding on both the loops and ferrites really helps reduce capacitively-coupled inductive noise sources, like power-line hum. Monitoring with my headphone amp, I can hear a very low-level hum and the occasional faint whir of a distant vacuum cleaner or blender motor. They are not even close to being strong enough to trigger a detection. We do have an underground electrical distribution system in my area, which probably really helps.
There is also a bit of low-level processor noise from the controller that I can hear, as my controller is about two meters from the antennas at the moment.
When I used to work 40 meter CW, there was a HV distribution transformer a few blocks away with arcing insulators that completely overwhelmed the band. A letter to the FCC with a copy to the Commonwealth Edison engineering department got the issue solved in a hurry.
I used a transistor radio tuned to a dead portion of the AM band to locate the arcing transformer insulator. That might work for you. The ferrite antenna in the AM radio makes a pretty good direction finder.
Some area have Internet over power line distribution (BPL) that can really kick up lots of interference.
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