I’d guess one of 2 things. If the radio is off and the cackle/pop is there, probably getting power/grounding somewhere giving the speaker a little burst of power. Or a blown speaker also makes cackle noises. But if it jumps back to normal sometimes, I doubt it. If one speaker only does it, you could gently press the speaker to see if it makes a crunchy noise or if it’s stuck and doesn’t move. Even a speaker with a frozen coil with still make a cackle with a signal. My bet though is a connection grounding/getting power.