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It sounds to me that your turbo rotor is hitting the housing, not insufficient bearing lubrication. You can check this by removing the intake hose to the turbo and spinning the rotor by hand to see if it is hitting the housing. Do this while the turbo is cold naturally (duh). At the same time you can also check for lateral play in the rotor which will give you some measure of bearing wear. In either case I would suggest replacing the turbo cartridge. If the rotor has been hitting the housing there is probably some deformation of the rotor blades and perhaps some metal missing from the blades. This is a problem that should corrected at once as the metal has to go somewhere. If you are lucky they are in the bottom of the intercooler, if you are not lucky they are in the engine. If you have a Mitshubishi turbo and are fairly savvy you could replace the cartridge (and oil supply line) yourself in an afternoon.