Problem with metal shavings in fuel filter housing.

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I have a 2009 VW Tiguan 2.0TDI. After a recent recall for a fuse and counterbalance, our tiguan started to randomly stall whilst driving. It was like no fuel was getting through to the injectors. After inspecting the fuel filter housing it was noted that there were an excessive amount of metal shavings around the top of the fuel filter housing. it was cleaned out and the problem when away temporarily. The car hasn't stalled since but this time the o ring at the top of the housing has gave out and yet again more filings were found on the inside of the housing and in the engine bay. I would like to know what might be causing these filings, where should I start to look