ErnieBoy, could you please share the .skp file? (Also which version of V-ray do you use?) I'm not sure what causes it really.
It could be overlapping geometry, shadow or reflection of a hidden geometry, weird lighting positions/setups, environments/fog etc...
If you're using V-ray 3 , Denoiser feature is a good solution for noise issues: https://docs.chaosgroup.com/display/VRAYSKETCHUP/Denoiser