Does anyone else's headphone output sometimes emit a sort of low-volume, high-pitched noise sometimes? Mine does and so does my friend's machine. Both purchased in last few months, but a good 2months apart. It occurs both with headphones, or when plugged into an amp / mixer. Sometimes it happens when no music is playing back. At those times, playing a couple of seconds of music and stopping again clears the problem - for a while. Any suggestions?