Keep listening , and see if it stops for morning tea ..if it's manmade it will stop sooner or later ....

Here in CHCH NZ , we have sweeper trucks that emit a whine/humm that can be heard for 10 miles in calm night conditions ...cant hear it outside cause the noise of nature blocks it , the silence of the bedroom is another mater .......hummm mmmmmmmmmm mmm mmmmmm drives ya nuts