We have an automatic thermostat controlled attic fan that makes a low hum that changes/goes away in different parts of the house. Do you have one powered by some breaker that's not getting turned off?
Is the hum caused by the roar of the burner in your gas water heater?
Could it be the wind blowing through your home's crawl space that's allowed due to the outside wall ventilation vents in the crawl space?
Do you have automatic outside sprinklers that may be coming on ... sometimes water running in pipes under the house as they feed outside faucets/valves can almost make a humming sound in the pipes?
Is the step-down power transformer on the power pole that feeds your house humming?
