Select sine wave only, start at 90 "PO" (oscillator phase shift). Play a note (for lower note, you'll hear it best). Then 3Osc starts with a click.

Should this be fixed? If you have an attack time of > 0, there is no additional fade in algorithm required. But if your attack time is = 0, I think a fade-in algorithm must be applied.

For background info, zynaddsubfx uses S-formed envelopes for fade-in. The length depends on the note played, which results in a punchy, but not clicking sound for all frequencies.