To be sure the sound you hear is mated to turbo activity, I would first get a boost gauge for it. The tachometer sort of echos turbo activity, but not accurately or simultaneously. For instance, you'd hear it spooling up an instant before the power comes on.

Generally speaking, the sign of a failing turbo is oily smoke from the tailpipe on pull-away starts, which then seems to clear up a few seconds later. This is caused by pressurized oil misting past the turbo's rear shaft seal and into the tailpipe where it gets burned. A whining turbo is not necessarily indicative of a problem, but on the other hand these turbos are normally pretty quiet. Another thing to do is to pull off the pressure tube from the compressor side and spin the shaft. It should spin freely without lot of play.

Today's turbos are managed by the ECU so they run cooler than their counterparts in days of yore, when you'd be happy to get 150K out of one. Today they go 300K and more according to my indy tech, who reports he's had to replace exactly one since the 850 was introduced - in a car with 275K.

The key to maintaining a turbo is do all your oil changes on time, use synthetic for better flow characteristics and thermal properties. Most on this board would recommend Amsoil or Mobil1.
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David
1998 S70 T5SE // misc mods (mostly lighting) // red calipers
1992 940 GLE // Hella Micro DE foglights and 170K