It could be a manifold leak.
But, it also could be the Turbo itself.
Take the Car back to where you had the Turbo Rebuilt and see if you can get them to take a test drive with you.

Engine not running grab the Turbo Compressor wheel and push it back towards the Bearing Housing and rotate it to see if it is dragging on anything.
Do the best you can to pull it back towards the Air Inlet and rotate it again to see if it is dragging on the Housing.
If there is any dragging there is too much play in the rotating assembly or there was not enough clearance between the wheels and the Housings.

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84 300D, 82 Volvo 244Gl Diesel