I second something less than an airtight seal between the turbo and the intake manifold.

"I rebuilt the turbo" makes me nervous. The rotating assembly has to be precisely balanced because it spins something like 30-50K rpm at full tilt. If you moved the turbine wheel relative to the compressor wheel, it's possible the imbalance is causing one of the wheels, likely the compressor wheel, to scrape the compressor housing. Pull the crossover pipe and make sure there aren't aluminum shards getting to the engine. Pull the turbo air inlet hose and check that the turbine blades aren't shinier or sharper than you left them.

Sixto
87 300D