When I am in reverse I hear a flutter noise that increases as i put the gas on. The car also surges a little bit (not rpms) as it backs up. Any idea what this could be?

1993 300E 3.2
156,000 miles

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1995 E320 Sportline Wagon "Bernard"
black on black leather

http://i.imgur.com/BdZ7jM3.png