the sound is turbo blowback... ie you are under boost, the engine shifts or you back out of the peddle and the boost in the intake surges backwords through the turbo. Fairly normal with open intakes. The stock intake helps hide this sound. Compressor wheel wear/endshaft play can make the noise louder, but all fixed vain turbos has surge or blow-back noise.
And again, you don't have a blow off valve
You can unplug and/or remove the stock back pressure valve if you would like. But you won't have heat in the winter for 20+ minutes...
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Last edited by Bones; May 3rd, 2013 at 10:59 AM.