This is an interesting phenomenon. I wonder how many people have heard these sounds and just dismissed it as a plane flying over or some other industrial noise. Maybe that is all it is. I noticed in most of the videos there is a lot of cloud cover or it is overcast. I did laugh a little in the first video at about 59 seconds when the bird flies by you can tell the guy filming thinks the bird is making the noise for a split second. These sounds have a resonant quality to them. With all the different frequencies we have blasted through the airways it would not surprise me to find out that the frequencies are aligning and producing resonance. My first thought when I saw this was that it reminded me of the trumpet blasts in the book of Revelations when the each of the seven seals are opened. I’m sure we will find out what they are someday.