Subtle high-frequency noise is normal for Corona and other path-tracing renderers, especially at the beginning of the rendering process. But strong noise that won't go away after many passes, or after a long rendering time, may suggest that the rendering engine encountered some problems in your scene setup.
If you are certain that the noise problem is caused by a bug, please report it to us.
For possible solutions on excessive noise, see: How to get rid of excessive noise?