Teaching you to suck eggs I know but between transmissions you won't hear the crackling because it's below the squelch level... but you knew that.
OK so you've turned the squelch off and still note the crackling when carrier present? I know a radio that does that. I think it must still have an element of AGC regardless of the squelch, so when it detects carrier it turns the gain up. With no carrier the background is actually noisier, so it turns the gain down.
Is the crackling consistent with 'atmospheric' background noise? Or static? Or distant thunderstorms? Or is it a soldered joint inside the radio? A good whack to the front panel might teach it a lesson.
I wish I knew a radio engineer who could fix this stuff. Good luck, David.