I just got a used Fold Processor today, and had the same question (and a Google search led me here).
When I turn it on, I also think I hear a very quiet whine getting higher (like a capacitor charging up or something), but only when the inject knob is near the center (and it happens even with the fold all the way down).
Folding all happens properly, as does the division. Inject seems to work properly as an offset, just the whole thing adds a bit of noise.
Mine's used so it might be from misuse too...
Any ideas?