The M5 does suffer with vanos noise - its just one of those things. there is a lot of "myth and legend" surrounding vanos but because its noisy does not mean its not working. Its usually always worse when the engine is hot and around idle speed and is at its worse when the engine is blipped and the revs fall back past ~1200-1400rpm.

If it hasnt thrown any codes then its probably fine and an oil change with the correct oil will help, although as the oil ages the noise will get louder again (think of it as an audible service indicator ).