Yeah, so enabling Defer Atmo will help a lot with the noise, but may increase render time (any solution to the noise will do that however). If noise is still an issue with Defer Atmo enabled, then increase AA. I would not go above 6 for an atmo-only render. If noise remains at AA6, try increasing Cloud Quality. If these are v3 clouds, you can also try increasing Voxel Scatter Quality, make sure it is at least 100, and you can try up to 200. However as far as I know this should affect edges much less or not at all, so it would be more for noise within the shading of the cloud volume (e.g. shadowed areas).
- Oshyan