Had the exact same scare.... my 2011 VW Jetta Sportwagen TDI, had same rattling noise, scared the crap out of me. Found out it was the exhaust. danged 2 bolts that mounts on the exhaust system just behind the TDI motor just came off.... due to diesel motor vibrations. Going to buy bolt fastener ... and hopefully keep it on!

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Nicknamed as 'MJ'

Owns:
2011 VW JSW TDI, 199,000 miles of 12/3/15
2 of 1989 Mustang 5.0's.

Looking for an MB 300SD Turbo Diesel

owned:
1971 300SEL 6.3, 1973 280, 1991 300SE. 1988 Mustang LX 5.0, 1990 Mustang GT, 1991 Mustang SSP, 1992 Mustang GT, 1970 Mustang Grande 302, 1994 Volvo 940 Turbo and 1981 Volvo 240 DL. The rest are not worthy mentioning.