More than likely the lower control arms. From what I understand it's a common problem. Mine was doing the exact same thing. Very easy DIY! http://www.xoutpost.com/articles/x5/s...trol-arms.html

Hope this helps!

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2005 Z4 3.0i, Sapphire black over Dream red, 6 speed, ZPP, ZSP, heated M-sport seats, Dinan upgrades (ram air duct, high-flow intake, free-flow exhaust, strut and supplemental braces)