Noise makes it harder for a codec like H264 to work well. To be effective there needs to be portions of the scene that do not change. When you have movement or a lot of noise there is nothing in the frame to frame that is the same so the compression has to work harder and things break down faster. Add saturation and sharpness and you are in the Danger Zone pretty quickly. The goal is to always look for ways to help out the compression, going soft, less saturation, lower ISO all help.
I just got that lens also and love it, it best lens for 90% of the time. Would love to have 2.8 at times and I do have NIkon primes for that but the range and IS make this just a fantastic lens for stills and video.