UFO's and the sounds

UFO's have never really interested me. Don't get me wrong, should something significant happen in regards to them, like one of them landing in the middle of the field during the Super Bowl and aliens stepping out and zapping everyone, I would be as interested as the next guy.
However, these constant tales of eye witness encounters... What do they add to the puzzle? About 58 gazillion people have by now reported seeing a "strange light or craft", and as exciting as I'm sure that is for them, just another reported sighting adds very little information.
I must though, report that the sightings in the past few years have increased dramatically! At least judging from what I read at my different paranormal sites.
Something may be abrew.
Another thing that might possibly have to do with UFO's are the sounds, and this I actually find very fascinating and equally disturbing.
From all over the world there are reports of strange sounds. Some of them are loud clanking, banging sounds, some are sounding like cannon shots, some are just a deep, vibrating hum that makes everything shake, and some, and this one I've heard myself, twice, are like a massive jet enegine combined with a strange pulsating.
This is a link to it, but it wasn't as loud as that when I heard it:
http://youtu.be/X4DpHkWuOmQ
Now as far as I can tell this it not a sound I have heard before. Nor can I even begin to imagine what is causing it, but I'm not saying it's necessarily paranormal in nature.
Of course I have discussed it with others. One gentleman in his 60's was actually with me the second time I heard the sound. This time it was daytime and the sound was heard for at least an hour.
We discussed what on earth it could be and we both agreed that the pulses sometimes sounded like an old train in slow motion.
I became aware that there are actually two different pulses, one slightly faster than the other so that sometimes they overlap.
I suppose it could be some kind of heavy machinery, but at all hours? The first time I heard it it was night.
Also, why are people hearing the same sound all over the world? And why now, in just these past few years?
And oddly enough, right after the first time I heard the sound, no more than a week or two after, the bird deaths started. The mass bird deaths that have been happening all over the world.
The second one that got headlines was here in Sweden.
Coincidence? Maybe. What do I know? I just know that I don't have enough parts to make a puzzle piece out of this one yet. But it's on my mind as "something".
Now if these sounds are not UFO related, could they be created by secret government projects?
Of course they could be, and if they are there might still be some UFO tech involved, who knows?
Could the sounds be coming from the earth itself?
Another possibility. But there are just theories, just thoughts.
The sounds are a mystery, and a mystery is a usually a part of a puzzle piece! :)