Sounds like a bird tweeting when the boost is in/past yellow. Like a whistle. I just picked it up from having the flex pipe replaced and now I can hear it.

What could it be? No smoke on startup, no smoke while driving, boosts fine, holds boost, just makes this odd sound.

Have you checked the diverter valve?

No, I have not. Nor do I know what it looks like....

Is it a constant sound when under boost or does it do that only when you shift?

Mine whistles under boost due to the delivery pipe silencer being removed, but it's a very minor whistle that is not annoying at all.

I have also heard on a few occasions a very minor peep, or tweet, when shifting under boost. That has happened maybe 5 or 6 times, and I have never been able to pinpoint it.

Constant under load. I haven't looked at it too much yet. It's pretty noticeable with the windows down, but I didn't think to roll the windows up and see how it was...

The diverter valve is in the front of the engine bay between the engine and the radiator. The stock DV is black plastic and is attached to the air intake going to the throttle body and a pipe going to the turbo. There is also a single vacuum line attached to it. The stock DV has a diaphragm inside and will "hoot" when broken. You're not really describing the hooting sound I'd expect from a broken DV, but it's a place to start looking.



My 9-5 has a very noticeable turbo sound when driving. It's a shhhh sound under moderate load and a high pitched whistle at full throttle. It can clearly be heard from inside the car. Is it possible that fixing the flex pipe has just changed the amount of normal turbo sound you're hearing? Do you remember what the car sounded like before the flex pipe needed replacing?

It was always pretty quite before the flex pipe busted. I'll try to get a video of it today.