Originally Posted by jdbretz
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.
Not going out, just not perfect and making a little noise. Most people aren't turbo folks and chalk to up to normal turbo noise. A lot do it, some don't. Increasing the boost pressure significantly increases your chances of it happening.