Originally Posted by Vargasturbotech
Turbos should not I repeat not make that noise, a whistle is 100% normal and with less restrictive exhaust you will hear it more. With a siren or as its called turbo howl your turbos are well on there way to going out. The noise is caused by excessive play in the center section, either in the journal bearings radial play or in the thrust axial play, usually its a combo of both. This allows defelction under boost as the exhaust imparts force on the turbine wheel, the excessive play allows the compressor wheel to actually slightly contact the compressor housing causing metal to metal contact and this noise. Once this occurs it actually starts to machine away the blades of the compressor wheel a little at a time and it will eventually throw itself out of balance, take out the piston rings and everything else with it, and you will start smoking badly. Another thing I have seen with stock turbos is, the turbine side piston ring allowed excessive oil to escape past, which gets under the heat shield and from the extreme heat turns into carbon and as this builds up under the heat shield it pushes the back of the heat shield into the back of the turbine wheel, this can also cause this noise with the same results. Bottom line, turbos are on the way out, next thing in the time line is smoke. Hope this helps.
Wow this sounds pretty bad. But honestly I've been a member of this forum for awhile now and have heard about this siren noise way in the beginning already on 2007 models years ago. If this is in fact so bad, why haven't we heard of any cases of turbos failing yet? This poll points to the majority of users having this noise and multiply that by 5 years worth of 335i users tuning their cars with the majority having experienced this noise for years, yet pretty much no one has had their turbos fail yet from this noise. I'm sure if we dig hard enough there might be 1 or 2 cases but those cases can have other variables as well that caused it. Perhaps its not that bad?
FWIW, I have this noise and it went down alot when i took out the tune or when I installed the stock air box. Do both and I'm pretty sure its quiet again.