Sorry to wake up this OLD thread 11 years later.
That's very interesting. I have the same problem on an Indigo I just picked up really cheap. The constant white noise is independent of the volume level. If I put my headphones into the main left out, the synth sounds perfect.
The headphone jack is mounted on a small daughter pc board. It is fed the main output signal from the main board and runs it through its own amplifier. Clearly the main output signal is just fine so the problem must like in the headphone amp board.
I'm going to ask Access service if a replacement board is available.
Had anybody else seen this?