Screeching from the turbo is usually heard under boost.  If this is the case, there is an imbalance or clearance problem with the turbo.  Improper balance causes wobbling of the turbo shaft.  Excessive clearances can be caused by worn or damaged bearings.  When bearings are out of spec, turbo shaft wobble is possible.  In some cases, the bearing play is so excessive that turbo wheel to housing contact happens.  If the (compressor) wheel contacts the (compressor) housing, the metal-on-metal contact starts to shave metal particles and produce the high-pitched screeching.  If the turbo is confirmed to have too much play, a turbo rebuild is necessary.

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