I have been experiencing crackling noises from multiple speakers. It is a LOT worse from the rear surrounds than it is the front, but there are occasional crackles from the front as well.
I actually tried connecting the rear speakers to the front channel, and most of the crackling went away. So this seems to eliminate a speaker or wiring problem.
One guess I have is maybe the receiver is underpowered, and the crackle is due to "clipping". However, it even happens at very low volumes.
Another guess could be dirty power, but it's happened at two different locations. Also that wouldn't explain why the crackling is much worse in the rear than front.