This is the appropriate fluid, but don't think any fluid will help the noise:
AMSOIL Severe Gear Extreme Pressure Synthetic 75w90
(Product Code SVGQT)
I had an 06 CTS-V for several years (sold it in 09) and that model had many TSBs on diff whine. Many Caddy Forum members tried heavier weight fluid (e.g. 75w110) but I never saw consensus that this really helped and the increased viscosity would result in a loss of net HP and a lowering of mpg.