GPU and CPU volumetric render noise discrepancy Below are a couple of images comparing quite a big discrepancy between GPU and CPU volumetric noise.
Same settings, much more noise on the GPU render!

Has anyone else experienced this? 

The GPU must have a harder time clearing the noise for some reason at same Sample size..





The original images are on Google Drive.
There are some differences between CPU and GPU rendered images. Some have been fixed in 2.78, some have not. Maybe later. You can do nothing about this. 
Only one recommendation: use what works better in your case and don't combine CPU and GPU rendering for animation. :-)

Blender 2.78 bugs fixes