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use a small portable AM radio that has an internal antenna only.
These ferrite rod/coil antennas are quite directional.
Walk around with the radio to find the source of the noise.
It may be a light dimmer (but you said you turned OFF (not down) all
electronic stuff. Or a computer. Or fluorescent light.
I like the idea above to go to the fuse box and shut off all power.
If that works, turn on circuits till the noise returns. That should give you a hint.
If the noise is there with the power to the house off, suspect a noisy transformer
on a near by power pole. Looks like a trashcan on the pole or a paint can.
If you think that's it, call the power company. Good luck.
Last resort, check your neighbors :-)
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