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      11-13-2012, 09:25 PM   #9
jdbretz
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Drives: '07 E92 335i, '07 E70 X5 4.8i
Join Date: Dec 2010
Location: Florida

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I believe the cause to be the turbo bearings going out. To the best of my knowledge (which is somewhat limited on this topic) when they go out the shaft then has a little play in it which as it spins causes the noise. I may be incorrect but that is how I understand it. I just recently got the siren noise after running the guts out of my turbos at an airfield event. They still make and hold boost, but they have the siren sound now.