My Jetta TDI whistles as it spools up then "hisses" when the wastegate opens at full boost. It sounds like yours is hissing the whole time during acceleration, eh? And you have normal boost, it sounds like? Strange...

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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)