May have heard a UFO

Well first of all before I tell you all what happened I have no proof what so ever that this happened, but I would like to share it with you anyhow and would like to know your opinions.

I was speaking to my friend via skype about a month ago now and we were just chatting about random things and listening to music, then out of no where I could start to hear some kind of humming noise from outside over my music, to begin with I thought nothing of it, but then after a while the noise started to freak me out somehow, so I stopped my music and was freaking out saying to my friend "omg what the hell is that noise?"
At the time I wasn't really paying attention to what my friend was saying I was just looking out of the crack inbetween where the two curtains meet, the sky was a tad bit brighter than it should of been (maybe the clouds had allowed the moon light to shine through but not quite sure).

Then the humming noise sounded like it was decending, the noise it self was scaring the hell out of me because it just didn't sound right at all. When i heard it decending I put on my headset and he started saying "what the hell i just heard a noise what went like this (he made the noise with his voice and it sounded just like what i heard). Then all of a sudden the noise just stopped.

Once it had passed we were both freaked out, straight away we started trying to debunk what we heard, for starters we live about 4 hours away from each other, it couldn't of been a plane because of the sound and also they arn't that loud to reach that far. Also we fully disproved that it wasn't a plane the next day because when we were talking again i heard a plane, it didn't sound anything like what we heard the night before as well as he didn't hear the plane when i asked him if he could hear it and he could not. There was also no way that it could of been a helicoptor either because that too didn't sound anything like one.

The things that creeped me out the most is how reacted to the sound as well as how my friend could hear it as well.