stupid as it may sound my sister had the same problem with her audi... well wasnt a problem but she just didnt know it was a turbo... being a badge buyer she never even looked at what engine it had,
Id check the condition of drive belts as when they are wearing out they can make a whistle/spooling noise, dont lube the belts, makes it worse
I don't experience it on my 300, but on my way to work almost everyday there's this old Toyota Previa van (the one that looks like an egg) commuting the same way I do, and it WHISTLES! Seriously! The tail pipe on it must have a British police whistle stuck in it. You can't hear the engine or anything from the van. Only the whistling sound. It's actually a kick. People smile when it drives by. It's like a cartoon the way it looks and sounds.