I have the same issue on the right channel. To my understanding it is the digital noise leaking to the analog section of the Play. It is present also through the RCA output. The right channel is closer to the digital section, therefore more prone to the interferences. Burson made few revisions of PCB board addressing this issue, I guess with the better shielding the digital section. In my experience, Burson is very quick answering the e-mails, they might ask you to send them the picture of the board.