Yep, what Terry said. Every power steering system I've ever had would moan and sing if it had air in the lines or was low on fluid. Not sure about our W210s, but many PS systems self-bleed with a few lock-to-lock steering movements. Try that and check the reservoir a few times -- if nothing else, it doesn't cost you anything to peek!

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI