Is your external input tied to ground? On a sammichSID, the input is a switching jack, so if there's nothing plugged in, it *should* tie to ground automatically, if it hasn't developed a soldering fault. If there's anything plugged into that jack....unplug it!
It might also be the VCA leakage. I've noticed that some patches tend to leak worse than others.
A bad/noisy power supply can also cause noise issues.