It's definitely not the neighbors' central air-conditioning systems. A neighbor directly across the street from my house installed a geothermal heating system and was wondering if that could be the source. Do geothermals run 24/7?
I am not sure what to do next. Are there any kind of engineers or other professionals who have equipment and who can help me solve the mystery? How do I find such a person?
Answer: I can offer nothing definitive, although an Internet Q&A site called "Green Building Talk" says this:
"A correctly installed geothermal heat pump should not be noisy. I have been next to some that are so quiet you have to put your hand on them to see if they are running. Most make a soft humming noise."
I can imagine how annoying this must be 24/7. I thought it might be something electrical until you said you continued to hear it after you turned off the main breaker panel. Perhaps one of our scientific minds might have a definitive answer for you.
Winter is a'coming. A safety tip from First Alert (you don't know how many times this warning is issued, and still people disregard it):
Never use generators indoors. In a power outage, use portable electric generators outside only. Never use them inside any confined area where carbon monoxide can collect. Be careful to follow the operating instructions closely.
In addition, refrain from using charcoal grills, camp stoves, and other similar devices indoors.
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