You probably have a leak in the manifold side. I just had/fixed the same problem with my w126. Turns out it was one of the large rubber o-rings between the compressor on the turbo and the cross-over tube.

They can sound really weird...the guy at my turbo shop here said he has even seen them whistle like a siren on an ambulance.

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My Primary Driver - '85 300CD - 4-speed conversion, 2.47 rear, lowered, euro headlights, rebuilding (not restoring so much)

Wife's - '08 Saab Sportcombi Aero

Riding a '03 Yamaha Warrior