Why would a speaker make a clicking noise after ~1 hour even when disconnected?
The speaker's transformer should clean any dirty power you might have. A gross under/over voltage in your home (more than +/- ~10% of nominal) would probably be evident in that you'd have equipment failing left and right; send 'em back to China.

It could be some Bluetooth funny business going on, or a bad DAC, I'm not sure. But if you cut the power and it still crackles, you definitely got a lemon.