I'll be able to answer your better tonight when I get home from work...

But in the meantime, I have some questions for you:

There are two types of Environment Fog-Simple Fog and Physical Fog. Which are you using?

Also, just an FYI, Environment fog cannot cast shadows, so if you want to cast shadows, you must use Light Fog
(just in case you are wanting shadows to better see the noise detail in your fog.... )
Just thought I would add that, as I don't know much of how you setup your lights....

Also, from the docs, in case you might have missed it:

"To create variations in the density and color of environment fog across the camera's view (like you are doing with your noise texture), turn on Use Layer and assign a texture to the Layer attribute. (You may need to limit the Depth Range, and increase Volume Samples for the environment fog shape to see the effect.)"


Also from the Maya 4.5 docs: (I know you are using 5, but try it, it couldn't hurt )

"Grainy or flickering environment fog troubleshooting:

Turn on Shading Samples Override for the environment fog shape, and increase Shading Samples and Max Shading Samples (to the lowest value that produces acceptable results).

or

Turn on Volume Samples Override for the environment fog shape, and increase Volume Samples (to the lowest value that produces acceptable results)."