Unfortunately it's impossible to rule out normal causes before we jump to UFOs or HAARP
Sky noises are one of those persistent topics that crop up around the world without any real answer. These noises are often low, loud roars or hums. The noise seems to come from everywhere, with no apparent single source. And the noises often have an oscillating quality reminiscent of a dubstep bass line.
These noises are clearly frightening to people who don't know what they are. But so far, the noises are just that - noises, and not precursors to some scarier phenomenon.
Explanations frequently range from "UFOs" to "HAARP experiments." The problem is that there are so many real, known explanations for noises that can seem to come out of the sky. And unfortunately with a sourceless noise like the ones that witnesses report, it can be very difficult to rule out any of the normal causes.
I live within the flight zone of a naval air station, and I am quite accustomed to the sound of various fighter jets flying in a variety of altitudes, times, and weather conditions. There are times when the jets can sound pretty weird. The sound often seems to come from nowhere, and can have a strange, broken quality to it.
Think of aircraft noise as being the auditory equivalent of a boat's wake. If you're standing on the shore, you can't always tell that the waves you're seeing are from a wake. A boat may zoom past long before the wake reaches you, or it may get chopped up by the other waves in the water (equivalent to atmospheric disturbances).
In other cases, I suspect neighborhood pranksters with a big stereo. It wouldn't take much to cook up a few odd, loud noises, then broadcast them out your window to the neighborhood at large. In fact, some of the sky sounds closely resemble the testing CDs that people use to test the volume and frequency range of their car stereos. Put a booming car two blocks away and stick in one of those CDs and presto!
It may not even be the work of pranksters. It could just be kids doing what kids do, which is "weird stuff for no apparent reason."
Mining, drilling, and fracking operations can also cause sounds that resonate strangely for a long distance. Not to mention everyday phenomena like cars with tires that have a weird wear pattern, someone practicing oddball music with their garage band, distant thunder, rescue helicopters passing beyond visual range, and more.