Interesting topic....seems ageless and never ending too, Noise...AC ..hum...and hissing.
Yesterday my tracking was shutdown due to some new AC humming.
My guess is window ac's all running and I recently added a 4th one near the studio room/closet.
Id replace the central air but its $10,000 to $20,000 these days for a new one, mine are so old repairing is probably tossing money away.
Window units are $250. Disposable. Im learning to go without too, and only have them on at night. Its amazing the body re-adapts to natures climates.
for AC hum and having so much crap on the line, let alone the whole neighborhood who knows where it all comes from.
Also transformers seem to be ripe and Ive recently added some wall wart transformers AC to AC psu.....working in a closet, small desktop setup the SM58 with transformer 24 inches from the ac strips and wall warts and each units transformers for the internal transformers power supplies makes it worse.
Ive got a large room with high ceiling sitting empty, maybe I need to follow your findings and "relocate"? I really like the small desktop setup though its prone to picking up noise in the mics. (and I just ordered a Neve design preamp with a wall wart and 3qty transformers inside it!! omg?)
add> my noise today was the laundry dryer motor..fyi..