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It's either an inverter, a solar controller, or a noisy SMPS. Almost certainly in or very close to your home. Shut off your house breaker and see if it goes away. If it does you can turn them back on one at a time until the noise starts up again....at that point you know which circuit the noise maker is on and it should be easy to find. If shutting off the house breaker does nothing then the noise is from one of your neighbors.
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