There appears to be a bug in cos~ that makes certain parts of the output waveform rougher than it should be. It may also be present in osc~. I'd guess it's a problem with the interpolation. It's somewhat hard to demonstrate clearly since the distortion isn't very obvious, but it doesn't happen in Vanilla. There's no audible distortion on sinusoidal tones, but it can cause problems for some kinds of waveshaping. See the attached patch. cos__bug.pd