Did your LLY not come with a high idle from the factory? My factory high idle brings RPM to about 1200. Are you able to change yours to that? It might just be 850 RPM is not a good one to set the idle to. I also don't think 850 is going to help it heat up much. The engine needs to be under some load to generate heat, not sure 850 is enough of a load.
Do you have a factory exhaust? Mine sounds nothing close to that raspy, it's more mellow with a slight hint of the turbo whistle at 1200 RPM.
My engine bay is noisy, but not raspy, it's also a consistent mellow sound.
2005 LLY 3500 CCLB 215,000 miles