Just wondering, have you noticed any power differences?

I'm wondering if the wastegate could have been not working properly, and somehow "fixed itself" and you're getting decent turbo boost again...just a thought

Or is this on top of the normal turbo whistle?

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999