Chosen Solution
Left speaker started crackling on my 16” MacBook Pro. It sounds crazy. I restarted macOS, installed last updates, went through macOS diagnostic (no issues found afterwards), reset NVRAM, nothing helps. I don’t think its software issue, since it’s crackling even on startup screen sound. Mac still under warranty, but I’m not sure what’s going on. Maybe somebody faces this issue before
Not getting any errors running the onboard diagnostics doesn’t surprise me as the diagnostics can’t test the speakers as that is within the analog realm. I would focus on the sound preferences, altering the balance control so you bias the sound to either the left or right speakers for testing to verify things. If your system is still under warranty I would get it serviced by the seller (Apple) as the will replace the bad speakers and FYI Apple did have some issues with the speakers which it what your system is suffering. If you haven’t already you might want to get the AppleCare+ services ($$) so you have a service contract fo your system which will save you some money going forward.
