There appears to be an upsurge recently in reports of strange sounds coming from the skies ~ sometimes a low rumble, other times a whine, thump or a melody. Because the nature of the sounds vary so much, researchers are having difficulties pinpointing their origins.
According to Discovery News:
There are a few things to keep in mind about these strange, ambient sounds; for one thing, there is virtually no place on the planet where noise pollution is not a problem. We live in a constant sea of background noise, most of it unnoticed until we start paying attention to all the sounds and focusing on them.
Sources of indoor sounds are nearly endless, from faintly ticking clocks to air-conditioning to bubbling aerators in fish tanks. Outside the problem is far worse, with noise generated by countless sources including traffic; airplanes (seen and unseen); radios; lawnmowers and snowblowers; trains; highways; and high-tension lines.
Then there are the many industrial sources of noise and vibrations, including power plants and any factories with large machines such as auto assembly plants and printing presses.
For example, a flood of reports concerning a rumbling midnight sound in Malaysia eventually was determined to be caused by machinery tests a palm-oil company was performing each night.