Popping/Crackling speaker but not in the conventional sense

[dalaigh]

I have a mid 2014 15" rMBP. I had a memory fault recently that would cause the machine to reboot when certain addresses were used. I recently purchased a replacement logic board and installed it today. Everything went well, until I tested the audio and I realized that the left speaker was making a popping and hissing noise. A loud one at that. Convinced that I had somehow damaged the speaker in the installation process, I removed the new logic board and reinstalled the old one. The speaker seemed to work fine with the old logic board. I then reinstalled the new logic board only to have the popping and hissing reappear. I tested with headphones to see if it was a problem with the audio chip, but they work fine. I'm at a loss to be honest, having spent a fair few hours installing, reinstalling and reinstalling yet again.

Does anyone have any suggestions? I can find very little anecdotal evidence of this happening to others.

Thanks!