Wow! Six years old thread! That's some old horse you're beating.
..anyway, diesel turbos don't have wastegates. Some people are fooled by the variable vane vacuum actuator. Some amount of whistling is normal, but no-one can tell you whether your sound is normal over the interwebs.
When the turbo is truly a goner, you'll notice it, guaranteed. Of course at that point it might be too late and if it fails the wrong way, you'll have bits of metal in the engine, which is generally consider to be a bad thing.
If you know your way around the engine bay, feel if the turbo has longitudinal (along the axis) play. If you can feel any, it's on the way out.
If you don't want to get your hands dirty, take it to a turbo shop.
EDIT:oh right, overboost. That's caused by stuck vanes (mostly, could be crappy tuning software). Get your turbo fixed. You don't want it to break due to overboosting. It'll granade when it finally fails and then you'll have fragments in all the wrong places.
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DJ