I'm pretty new to manual mixture control, so forgive me if this is a rather dumb question.

I've noticed that at low altitude, having a rich mixture and an idling engine causes the engine to sputter, sounding similar to when the engines start to fail after a mechanical fault. This does not appear to occur above around 500-800 feet ASL. Is this damaging the engine?

Should I be leaning the mixture out at low altitude? I was under the impression that it's more common to lean it out at higher altitudes to keep the RPM up, but the engine certainly sounds displeased!