I had the same problem when I added a dac into my system. So, the culprit had to be the dac. I sent it in for a thorough checkup and the problem was narrowed down to a loose solder joint. After that was fixed, the popping went away completely. So, maybe you can try to check your components for any loose joints (it's no easy task). Hope this helps.
Cheers.
Cheers.