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My 135i had this siren sound. It only got worse with time. I was FBO and tuned since I got the car new and it wasn't until 15k that I noticed this. Soon it got so loud it was louder than the swoosh sound of the DCI. Always worse when engine was cold but still bad when hot. By 25k it was so loud it was driving me nuts. Still had the sound when reverted back to completely stock. Traded it shortly after that. Now my 2007 335i with 70k (untuned) miles is completley quite. Very faint spool sound when cold. I really think the tune and maybe the other bolt ons (DPs) took their toll. I wonder how long the RB's or Vargas would hold up to the abuse. Hopefully longer than the stock units. Are there any improvement to specifically prevent this syndrome in the upgraded turbos?
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