The 20mm is a great photo lens, it is not a great/fast video lens. In video, it is notably sluggish (compared to other lenses) at continuous AF, and rather noisy in its aperture stepping. If you're only experiencing this "grinding" in video, then I suspect it's just those characteristics of the lens not being a video optimized lens.when in picture mode it seems to be much quieter. It is in video mode that it gets loud and sluggish
If the audio is your only concern, then just dub over the sound in editing.ahhh... that was a fail buy! lol I bought this to use it as my main lens.