I actually just had the exact same issue with a brand new unit. Ended up having to return it for a new one that was not defective. I had bought two of these to setup as stereo monitors. Once worked perfectly fine, the other one had weird crackles over the audio.
After testing, determined crackle appeared when powered on, did not matter what was plugged in sound wise. Even having nothing plugged in, power only, still had the crackle.
- Different power cable ( possible defect in cable causing micro shorts ? )
- Different outlet ( used same outlet that working speaker was using )
- Changing location of speaker ( possible external interference ? )
Regardless what I tested, the speaker just randomly crackled just being turned on. Replacement unit worked fine.
Not sure what was going on there, but sounds like you had the same issue as me. Just a defective unit.