I did measure the voltage at approximately 6.3V and tried turning it down to 6.15V like you said you measured the SMPS output at. I thought that fixed it but then it came back again. I will leave it on for a day and check the voltage again after the popping starts. Do you think it is OK to turn the voltage lower than 6.15V? Someone had said when they were mistakenly running it at 5V that it sounded off.