Possibility #1
The sound like that of a jet flying too low could have been a lightning strike. What most people don't realize ─ or know? ─ is what causes the sound of thunder, even though most people will know that thunder always comes after the lightning flash, and the longer it takes, the farther it is away from you. But so here's the (possibly boring) science. :p
Thunder is effectively the sound caused by lightning, which itself is an electric discharge between a spot in the sky or on Earth ─ including water, under certain circumstances ─ with a high concentration of free electrons, and another such spot with a low concentration of free electrons. This electric discharge ─ effectively, an electric arc ─ moves
faster than the speed of sound, which is why the sound only comes seconds later. As such, thunder is actually a sonic boom, such as what you get when a jet crosses the sound barrier.
In the event of a supersonic jet, the sonic boom is the result of the sound waves in the air around the fuselage and wings of the plane colliding with each other and forming a shockwave. This shockwave travels with the plane for as long as it is moving at supersonic speed, and so different people on the ground will hear the sonic boom at different times, depending on their respective location and the plane's direction of travel. The same phenomenon ─ i.e. a sonic boom caused by an object moving faster than the speed of sound inside of the atmosphere ─ is also the cause of the loud sound that you hear when firing high-powered rifles (such as an assault rifle), and perhaps more surprisingly, it is also what causes the crack of a whip ─ it's the tip of the whip that moves faster than the speed of sound, i.e. ~342 meters per second.
Given that there were blue flashes in the sky, and given how similar the sound of a low-flying jet is to the sound of lightning, I'd say that the most likely cause of the phenomenon was lightning. The atmospheric circumstances would certainly have been favorable to that in these past few days.
Over here in Belgium we've been having a really bad storm over the weekend, with inland wind speeds of up to 120 km/h and wind speeds of up to 150 km/h at the coast and over the North Sea. It tore several large trees clean out of the ground with roots and all in the nature preserve behind my brother's house, knocked over the outer wall of the playground of an elementary school in my street at only some 80 meters from my apartment, caused lots of damage all over the country ─ the emergency call center couldn't keep up ─ and one man got crushed by a tree, albeit that it was his own fault. He had decided to go fishing. :rolleyes:
At the same time, the US has also been dealing with at least one tornado that I know of. Mother Nature has been very angry over the past couple of days. That could also account for the single lightning flash and thunder clap I heard this afternoon, because with a storm like that ─ and even with lesser intense storms ─ the howling wind strips off electrons from everything it passes over and through, even from the surrounding air itself. So you get an electric polarization effect happening, and then you get electric arcs between the negatively charged areas and the positively charged areas.
So, bottom line: if there were flashes of light involved, then I think it was probably lightning. ;)
Possibility #2
This one's going to be a bit longer and very technical, so bear with me. :p
Sonic booms have so far always been unavoidable whenever a plane crosses the sound barrier, and as such, the USAF (and possibly other air forces around the world) have already been looking for a way to avoid that by changing the design of the aircraft. This isn't easy, but it was recently in the news over at
Slashdot ─ it's a website for geeks ─ that a new aircraft design has been tested which isn't free of a sonic boom, but it does alter how the boom is perceived on the ground ─ it's supposed to be more "spread out" than that it would be a sudden shockwave. I have no idea what it's supposed to sound like, but it could very well have some weird sound effect similar to what you're describing.
Another but related fact is that the USAF has also for quite a while already been experimenting with hypersonic planes. The difference between a hypersonic plane and a supersonic plane is that with a normal supersonic plane, not all of the air rushing over the wings and the fuselage is moving at supersonic speed, even though the plane as a whole is flying faster than the speed of sound, while with a hypersonic plane, all of the air moving over each and every surface of the plane is moving at speeds greater than the speed of sound.
This effect is only obtained if the speed of the plane itself is in excess of Mach 5 ─ i.e. 5 times the speed of sound, or some 6174 km/h (~3836 mph). Due to the enormous aerodynamic drag associated with such a high speed, a hypersonic plane must be designed very differently to a normal supersonic jet. For starters, the wings must be a lot shorter, lest they would be sheared off by the aerodynamic drag, and the fuselage of a hypersonic jet is generally designed to provide enough lift in and of itself to keep the plane airborne at that speed.
Another thing that needs to be different ─ and here's where it gets interesting ─ is the design of the engines. A jet engine for a normal supersonic or subsonic airplane is actually a so-called turbofan, in which the center part of the engine provides thrust by continuously burning fuel in a high-compression combustion chamber ─ this is what causes the hot and sometimes flaming jet exhaust ─ while at the same time, a portion of the air sucked into the engine by the turbine at the front is channeled around the core of the engine, somewhat akin to having a propeller in a tunnel. The combined thrust of the clean air and the jet is then what propels the plane.
Now, when it comes to faster planes, this engine design is not efficient anymore, so for jets that travel above approximately Mach 2.5, the engine will be a turbojet, not a turbofan. This means that the turbine at the front will only suck in air that will be mixed with fuel and then ignited in the combustion chamber. But depending on the speed of the plane, that too may not always be efficient anymore, which is what leads us to the next step up, namely the ramjet. A ramjet works just like a turbojet, except that there isn't a turbine anymore. Or that is to say, the turbine is still there for when the plane must travel at lower speeds, but at higher speeds ─ as for instance is the case for the
Lockheed SR-71 spy plane ─ the turbine stops rotating and the intake of the jet engine is adjusted so that the air rushes straight into the combustion chamber by the sheer speed of the aircraft. Hence the term "ramjet".
But hang on, we're not done yet. :p Even the SR-71 is still a supersonic plane, not a hypersonic plane. And thus, the air rushing into the engines of an SR-71 is still doing so at a subsonic speed. For a hypersonic plane, this is different. The air rushing into the engines of a hypersonic plane is itself supersonic, which is why they call such engines "scramjets". And when it comes to scramjets, a number of interesting strategies have been adopted to increase efficiency and prevent damage to the engines from the continuous influx of air moving at supersonic speed. You have to keep in mind that at that speed, air is harder than concrete ─ well, you already know what air can do at much lower speeds, given the damage caused by the storm that I mentioned higher up in this post, and that was just at air speeds of 120 km/h.
And so, one of the techniques the USAF has been experimenting with when it comes to hypersonic planes is called
"pulse-detonation". What this does is that, instead of allowing a continuous combustion process and thus with a continuous exhaust jet, the air/fuel mixture is ignited in short bursts, resulting in a pulsed exhaust. And this generates a very characteristic sound, very dissimilar to other jet engines. It is also not specific or exclusive to hypersonic planes, because the infamous German V-1 used during World War II was essentially an unguided cruise missile that traveled at subsonic speeds ─ i.e. slower than the speed of sound ─ by way of a pulse-detonation jet engine, which gave it its characteristic rumbling sound, which does indeed resemble the sound of a helicopter somewhat.
Now, the USAF is certainly still testing experimental hypersonic planes that use pulse detonation, and given how fast those things are ─ hypersonic planes are usually intended either to carry a nuclear payload or as spy planes ─ they can cover a lot of ground in very little time. So that could easily have been it. :hmm: