There are several level of fixes, and there are many posts describing all the steps. One fix is to change the reservour screen, others add the official Mercedes additive Lubrizol or other additive, and all have different degrees of success.
The ultimate case is getting the updated power steering pump, and making sure the dealer or mechanic performs the air removal procedure..
My former 2001 ML 320 and my current 2005 ML 500 where very loud whiners until my power steering pump was changed to the latest part number and the air removal procedure performed. Now there is zero whine and only hear the V8 engine purr at low or higher revs...
There is a known design issue with the original power steering pumps, which causes them to begin whining loud. More even after they have a power steering hose incident and lose their power steering fluid.
Same as the engine valve noise, some people had very loud valve noises and the dealers say that is normal. Mine does not have engine valve noise (82,000 miles), after an engine head rebuild caused by an engine head defect at around 4,000 miles, and now I would have to open the hood and remove the engine plastic cover to clearly hear the valving sounds. Though valve noise is common, excessive valve noise is not and can indicate sludge in the engine, which is a very common problem in 2001 and earlier engines, or some other issue/tuning.. I do flush my oil regularly, though, to remove any sludge from my engine like I have done to all my engines.