Hello, on my two cars Hyundai i10 2010 and BMW x3 2006 I can hear a hissing sound when I open the throttle On the Hyundai it can be heard on low rpms and it sounds like a vanos valve(which I doubt that car has) but on the X3 it is much higher and can be heard at high RPMs (Full throttle).
This is normal. What you are hearing is the rush of air entering past the throttle body. You normally don't hear this because there is much less air passing the throttle plate at lower throttle angles. The sound of the vanos valve is a mechanical noise created by the mesh of gears where the sound you have described here is the sound of air rushing into the motor.
If both vehicles are running well without a Check Engine Light, then this is normal. This noise can be present with a vacuum leak, but a vacuum leak will be apparent under all RPM ranges. If your air cleaner box isn't secured closed, you will be able to hear this sound much louder and under a wider range of RPM's. If your were to remove the air filter box, you would hear this sound much more regularly.
The short answer is, this is normal. If your not sure, have one of our technicians take a look at your vehicles. It won't take much more than opening the hood for an experienced technician to know for sure.
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