Thats a clean out...take a wrench and turn to the left (counter clock wise)... Keep a bucket handy...insert a snake and then clean out the pipes...
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Current: 2006 E46 M3 'vert 6-sp 2008 E60 M5, 2011 E92 328 6-sp, 2011 E70 N55, 2012 E71 X6M
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