2000 volvo s40 intermittent whine...

[Phoenix]

Hi everyone,
I'm new to this forum and I gotta say... you guys really know what you're talking about! I mean I've seen replies to some issues that are far better and more professional than most of the mechanics I've been to! Which encourages me to post mine knowing that I will get the right answer...!
So the problem is (don't know if it's really a "problem"): I'm hearing an intermittent "metallic whining sound" from the back of my engine, somewhere near the turbocharger... can't really describe it cause I can't figure out if it's the sound of pressurized air leaking through a rubber hose or a metal wheel grinding intermittently against something.
It can be heard especially in idle, doesn't get louder when driving.... My mechanic said that it's a small air leak from the turbo but I don't wanna rely on what he said and end up with a major repair a couple of months later.

I know this description may be a bit vague, but that's all I can say about it for now, hoping someone could help me diagnose it better.

Thank you all for your support, and have a nice day!

[Phoenix]

Anyone?