Power loss and noise: I like the idea of a leak somewhere that is bleeding off the turbo boost. I also hear some rattling, maybe the air cleaner? An old trick is to put some rubber between the air cleaner housing and the valve cover, a broken air cleaner mount works great for this.

Turbo spin-by-hand - sounds normal. Does it have any axial or radial play? Should be just barely perceptible radial play, no axial play.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC