[ClampThurst]

2017 X3 N55

Hi all, Im experiencing a hissing/whistling noice coming from the engine when letting off the gas pedal, usially at 3-4k RPM at moderate torque and coming off. It lasts a second. Im wondering, is excess air coming off the turbo or is this the beginnings of the dreaded PCV failure? I do not experience any other noises at idle or at any other RPM range, it only happens when going from a moderate load on the engine to off pedal.

[mwat]

If what you are describing is what I'm thinking of I always just chopped it up to turbo noise. It's been the exact same since I bought my car, not worse or better so I'm fairly certain it's just normal surge or similar. Sounds cool, kind of like a BOV.

[BIG_KID]

Is your air filter and intake hoses secure ?
Aftermarket air filter ?
Might be turbo surge which is normal.
Aftermarket air filter will increase the sound

[ClampThurst]

Stock filter, all houses are secure. Im thinking it's also turbo surge. Where does the turbo purge its excess air? Is there a valve somewhere on the turbo or elsewhere in the intake to bleed off pressure?

[BIG_KID]

No, it purges around the turbine and back out the intake.
Sounds like a whoosh, whoosh, whoosh sound.

Think of the turbo up to speed (20,000rpm) and cramming air into the engine then the throttle blade slamming shut. The turbo is still spinning (somewhat) building pressure and that pressure can’t go anywhere (because the throttle blade is closed) so the air back flows and out the air filter.

Some install a blow off valve to dump that pressurized air in the intake manifold when the throttle blade is closed but most manufacturers don’t due to cost. The surge usually dosent hurt anything during closed throttle position. Open throttle surge is a different story…