During a space shuttle launch, in the seconds prior to T-0 the main engines ignite and take a few seconds to develop full thrust. The sound level builds a bit, but it’s a nice smooth white noise. At T-0 the SRB’s ignite, and the vehicle is released; within a few seconds a loud crackling noise develops and continues until the vehicle rises out of earshot.
What is the source of this crackling noise?
It isn’t just the shuttle, the Saturn-V sounded very similar.
My guess, and it is just that, so I’ll be interested to hear a more learned opinion is that in some senses it is sort of the reverse of thunder from a lighting strike. Lightning produces a nice mellow boom, with optional rolling modulation. This is because you have a linear source that extends across the sky, and the sound reaches you at different times from that long line - so producing a linear smoothing of the impulse, plus variations in air density, or turbulent winds aloft, will vary the arrival times, modulating the boom into a nice rolling thunder.
An ascending rocket is the opposite. Almost everything is swapped around. You get a steady source of massive pressure (the motors) occupying a point source, but the sound that reaches you passes through the exhaust gasses, which are very much hotter than the surrounding air, thus with a different speed of sound, and flickering away as a very turbulent flow. I suspect it is the flickering of that turbulent flow that leads to very fast changes in the acoustic path length from the motor bell, where the energy is emanating, to you.
Another factor, that may have a significant bearing, is the non-linearity of air. Very loud sound, especially when it is loud enough to really push the non-linearity of air, (and the shuttle certainly passes this test) produces a phenomenon where the high pressure edges travel faster than the rest of the wave, and the shape of the pressure wave becomes more and more saw toothed in shape. It may be that the large flickering pulses that emanate from the plume are naturally sharpened as they travel to you, leading to the distinctive cracking noise.
I very much doubt that this has not been studied and simulated, so it would be interesting to know the real answer.
it’s because the velocity of the exhaust jets from the SRBs is greater than the speed of sound. “mini sonic booms,” if you will. you hear something similar from fighter jets when they have reheat (afterburner) on.
Sounds plausible, although the exhaust plumes from the SSME’s is also supersonic, as evidenced by the shock diamonds.
they are indeed, but keep in mind that the sound from them is damped by the water suppression system on the launch pad. By the time the shuttle lifts off enough for the suppression to no longer have any effect, the SRBs are burning at full tilt.
I guess I’m assuming that the supersonic flow is the source of the impulse, but the reason for the staccato cracking is large scale modulation of that impulse.
I’ve stood not too far away from a Sukhoi Su-27 taking off with nearly ten Mach disks in its exhaust (air show, they do stuff like that then) and whilst very loud, it didn’t have the distinctive deep crackle.
Its the sound of the surly bonds of Earth being slipped.
I take it you haven’t experienced a lightning strike in extremely close proximity. It’s not a nice mellow boom but a sharp, crack akin to the crackling sound from the shuttle. The mellowing only comes from the greater time and distance to the observer.
If you can observe the flash and at least count to one before the sound strikes, it’s been mellowed, even just a bit. Trust me on this. 
Deal
The closest I have been I have counted to one. I live close to the top of a hill, but not at the top. (There is a golf course there…) I have seen a nice ionised trail dissipate in the road in front of me far too close to comfort, but was not even spare change compared to a real strike.
I heard it has something to do with Chuck Norris
I don’t think this has anything to do with it. I’ve been present at SRB and liquid rocket engine firings on the test stand and I didn’t hear any crackle.
So afterward, did you change your underwear or just throw them away?
Is the crackling sound heard when present in person at a launch or is it just an artifact of sound recordings of the launches? 
It’s the sound air makes when you uncrinkle it’s wrapping.
The sound you hear out of your computer speakers when watching a video of a space shuttle launch is not the sound of a space shuttle launch. What you are hearing is your speakers trying, and failing, to reproduce what a space shuttle launch sounded like.
Don’t think so. The crackling effect is still present even when the shuttle has climbed to high altitudes, at which point the ground-level SPL is not overwhelming.
It’s not about cheap computer speakers either: the crackling happens even when watching space shuttle movies at an IMAX/OMNIMAX theatre with a multi-million dollar sound system.
Is it perhaps ice? The fuel in a rocket is often frozen at very low temperatures. Moisture in the air freezes on the outside of the rocket while the rocket is waiting for launch. When the rocket is launched, the ice is blasted off.
Machine Elf is (mostly) correct about this. When a rocket is in the ground acoustic field (where the acoustics from the plume are reflected back onto the vehicle) you get a lot of shock interactions (places where two impinging acoustic fields interact to exceed the speed of sound. This is also true with nozzles with underexpanded flow and vehicles with multiple engines (which all current heavy launch vehicles have) due to interactions between the flows. All of this results in spiky fluctuations in the broadband acoustic field that sounds like crackling. Now, it is also true that microphone/speaker limitations can result in clipping of the signal, and for digital signals low sampling or encoding bit rates can create sound artifacts, but the crackling you hear during the STS (“Shuttle”) launch is real. The Shuttle, with three liquid engines and the two massive outboard SRMs is pretty much a perfect storm of acoustic amplification and flow interaction.
Stranger
I thought it was from contact with moisture in the air. I’ve heard crackling from a propane torch on a foggy morning.