This is caused by interrupted electrical current (audio signals) or, in other words, a loose or dirty connection. To fix crackling and popping, troubleshoot the connective wires to find the problem area and secure the connection and/or replace the cable. Hope this will help
There are a LOT of possibilities. But if this has not happened before, I would check anything you touched recently. If you have bare copper wires for the speaker with separate strands of copper that make up the copper wire... all it takes is a stray strand of copper wire to produce noises like you describe if that stray small bare wire touches anything metal. Move the speaker wires from the center speaker to the left speaker. Does the left speaker make the same noise as the center speaker: If yes, the problem is in the receiver. If no, the problem is in the center-channel speaker. Driving any speaker too loudly can cause damage that causes the speaker to make unusual noises also. If you have a bad amplifier channel in the AVR, you can use the pre-out jacks to send the output for the center channel to another amplifier or to an AVR you aren't using... play the sound through this new setup. If you still have the problem, it is in the preamp side of the AVR. If the problem disappears, you have a bad amplifier channel in the AVR. With audio progressing so quickly in recent years, it often doesn't make a lot of sense to try to repair an AVR, especially if the AVR is more than 5 years old. Getting a new AVR with all the latest bells and whistles may be the better choice for older AVRs. Or a used AVR that's newer than your older one might be worth considering also. audiogon.com is an interesting place to buy and sell audio gear. You don't hear of people on that site screwing each other with prices or not delivering something paid for in advance, etc.