With the sound fading, did you get a sense the source was moving away from you as it was fading? edit, oops, never mind you already answered that, sorry.
I have seen and heard military planes conduct some sort of exercises over my neighborhood and that lasts for a long time and is shockingly loud. The planes will just circle donuts up there for no apparent reason. But you can see the lights of the planes even at night. One time several planes sporting red lights chased one plane sporting green lights....or maybe it was the green lights were chasing a red light, oh me and my middle aged memory! But at least I could see something and know what was causing the noise in those cases.
Well, this surely is a mystery. Thank you for purchasing additional data capacity so you could share your account.
I am not sure but I wonder if your experience of the sound correlates in any way with the large solar flare that supposedly hit over the weekend. I have not paid close attention so I'm not sure when exactly it hit us but I know it was on my news home page over the weekend.
edit on
22-1-2012 by SheeplFlavoredAgain because: (no reason given)
You should really know that, considering the word in your nickname
) So, I went outside to see what was up, it was sounding from the north. After I realised it was no aircraft (the sound was too
constant in volume) I went around to the North facing side of my house. As I went round the corner the sound increased in volume somewhat... 
