Thats belt tensioner, might be worth checking the pully for free play.
When was the belt last changed? I could be justed streched or the tensioner could be worn.If it is the tensioner they aren't expensivehttp://www.vwspares.co.uk/product_info.php?products_id=118And the belts not much eitherhttp://www.vwspares.co.uk/product_info.php?products_id=624