The turbo bearing is lubricated from the engine oil, and the most common cause of failure here is a lack of lubrication. Deposits form in the oil feed line, and because of that the bearing does not get enough oil. A missed oil change and poor quality oil speed the process up on their own as well. The symptoms are a whistling or howling noise on acceleration, a loss of power or limp mode, and black smoke from the exhaust. Replacing the turbo on its own is not enough: if the oil feed line is not cleaned or replaced, the new turbo goes the same way. Prevention is three things. An oil change at least every 15,000 km, better every 10,000, with oil approved to BMW Longlife-04. A check of the oil feed line. And not revving the engine right after a cold start, because the turbo needs oil pressure first. Above 180,000 km a preventive check pays for itself.