Could it be a belt? I generally avoid the belt dressings, but they can be helpful in determining whether a given noise is belt-related. You can also use a candle and hold it against the belt as it spins... the wax will quiet things down without (in my case, anyway) harming the belt or letting it slip.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)