I'm going to put this one up here as a recommendation.
Raytracing produces the byproduct of noise. This is not news. Those not familiar with this conundrum will be indoctrinated very quickly when they try to trace their very first rays and are met with gratuitous quantities of grain.
In a still image, noise can be anything from charming to infuriating. On a sequence, it can be a disaster; invalidating what would otherwise be a perfect sequence worth thousands of hours of processing time. This has certainly been the case as I start to pull together the final frames for my upcoming technical short, Panigale.
Denoising algorithms exist, they're built into most raytracers by now. However these are not the silver bullet we may have been hoping for. In many instances they either create ugly artifacts that are worse than the noise to begin with, or worse still, create painful flickering or shimmering temporal artifacts. The latter is far more distracting, and far more difficult to remove. I have found myself manually painting out frames as if they were troublesome live action, far too often.
As a tool to have in your arsenal then, I highly recommend DEFlicker from the good people at RE:Revision. This tool is primarily built for stabilising high speed, time-lapse or stop-motion photography, however it also creates the rather charming epiphenomen of stabilising raytraced images. If you use the high-speed preset, with small details, then tweak the settings from there, you should be rewarded with pleasingly smooth images that otherwise were do-overs. Honestly what this plugin does is tantamount to magic, it seems to fix just about any temporal artifact you can encounter.
At $250 USD, it is far from given, but the amount of times this tool has saved my bacon, not to mention the capacity to have settings a little lower, should make this a worthwhile investment.