Yes, as msimanyi says, it could be dust buildup. Always a first step is to vacuum.
oops, I just re-read as I was writing this, but I'll leave the above since it's always good to clean the panels. But, now I'm thinking it might be capacitors that have reached EOL, End Of Life. If the crackling/pops appear with just being plugged in, then it's gotta be caps.
My Krell amp developed a static sound coming from one speaker when the amp was in standby. Standby wasn't "off", but just that the amp circuitry wasn't "on", or at least not fully, so for amplified noise to be sounding from a speaker was a bit shocking. Krell replaced all the caps, luckily under warranty.