Isx cm871 holset turbo hissing and whistling when letting of the throttle.

Just as the title says, when I let of the throttle, it hisses and whistles. Mind you, it is not loud, but it is beginning to drive me crazy!

It is a 2008 Volvo, if that has anything to do with it.

The boost gauge shows about 2-3 psi while it is doing it.

It has been doing it since I got this truck 2 years ago. It is still doing it even when I replaced the turbo with problems unrelated to this.

Is this normal? Or is it a design flaw?

Truck pulls well, good fuel economy. In fact, the turbo is quieter when pulling. So much so that I can barely hear it!

I made a video that you can listen to. Thanks in advance.
http://youtu.be/RgpG1DlqTew

 

Yes, it does it all the time when I let go of the throttle. It never used to bother me, but now it is driving me crazy! Lol