Hi,
The noise comes simply from the quality of the GND/5V that your computer USB provides.
Preenfms use directly this 5v so they're sensitive to their quality.
That's the reason why there's a jumper in the preenfm to disable the power from USB but keep the USB data.
Remove the "Usb5v" jumpter, and use a dedicated DcIn 9v wall adapter.
You'll get a clean sound AND the USB midi from the computer.
Xavier