As the title says, I have a step modulator pointed at the volume of an audio track. The amplitude modulation, when it happens, has zipper noise or crackles. All steps on the modulator are supposed to glide. By contrast, a modulated LFO pointed at audio volume is smooth enough not to cause any audible issues. I strongly suspect the artefact with the step modulator is what you would call zipper noise, that is a lot of smallish moves rather than a smooth enough curve.
Yeah that’s pretty much what’s happening. If you were to record or render the output & look at that waveform you see a straight line going up or down at each step. Those will generally not be harmonics of your basic frequency and instead introduce enharmonic overtones.