If nothin's hot, and it doesn't change by pressing on the breakers, I would say it's the nature of the panel, sometimes what it's mounted to makes it sound louder than it is, like the big board in the back of a piano. However if it's a buzz, or a fuzz sound rather than a hum [Linked Image]
The other thing is, what was on? The water heater, dryer, a/c, some other heavy load?
Things like this keep a person up at night.
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