I have always wondered why the sound that large rockets make during liftoff is not constant and occasionally sounds as if it's "popping"? Which seems to resemble the phenomenon of "rolling" thunder.

[This video](https://www.youtube.com/watch?v=OnoNITE-CLc#t=1m45s) for example (**Caution**, the link starts video at mid launch - so is **Very Loud**!)

I cannot see much of a visible indication that the rocket's exhaust is fluctuating. I would "expect" to hear a consistent rumble.

I kind of suspect the inconsistency of the sound is caused by interference of the multiple exhausts jets blowing against each other? 

Or possibly that the solid rocket fuel burns at a rapidly varying rate?

I cant imagine that the combustion rate of the liquid stage would vary by much, and definitely not produce intermittent explosions.

Any thoughts?