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
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?