It's possibly coming from the windshield gasket. On mine I had the same problem. I went and coated the rim of the gasket where it meets the glass with silicone sealant (clear). That solved the problem for me.

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2005 Audi A4 1.8T CVT -Silver/Black "Siegfried"

1982 300D - Silver/Blue "Ralph" -For Sale:
http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/119226-1982-300d-sale.html#post852260

1989 VW Diesel Jetta Blue/Blue "Bodo" RIP