Whining noise under boost

[Eyvind Spangen]

My newly-acquired '90 200TQ20v (Europe) have a strange noise under
boost. It sounds just like an air leak, but the pitch it much lower
than the usual turbo whine. Could it be the bypass valve leaking? I
can hear the valve working (a "dumping" sound) when I go off the gas
pedal under boost. The dash gauge only reads 1.4, so I guess I have to
check out a few things..

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Regards,
ES