Whistle in second gear only - recording attached

Hi all, I have a 2014 s9 5sp auto 2.5td which has a whistle (dentist drill like), but only in second gear. It kicks in as soon as it changes into second gear and dies out once it goes into third. It increases with acceleration but the noise is there even if I'm crawling down hill with foot off the accelerator. It's only in that gear.

Mechanic said that there was a crack in EGR pipe assembly and so replaced that, but the whistle is still there. Mechanic now says that he thinks the turbo needs to be replaced but can't really explain why it's only in second gear and completely non existent in other gears other than to say that second gear must hit a sweet spot.

Here's a recording https://drive.google.com/open?id=11wRUfDPEGvUvsmqGG1xELjOx9D9zDTnm

As you can hear, it happens in second gear, all of second gear and nothing but second gear.

Any ideas?

Logic would dictate that it's in the gearbox, if that's the case.

However, the mechanic is correct that certain conditions may only exist for the engine in second gear during typical driving. The drive train is at a specific ratio, and under normal driving conditions you'd expect a certain loading of the system and that could be creating noise. I don't think that's what's happening here, because of what happens going downhill.

My car would do it more often than not at a specific RPM under load because the gasket between the turbocharger and the exhaust manifold had deteriorated, causing exhaust gas to leak past and make the gasket behave like a reed in a clarinet. It wasn't any good at jazz, so I had them fix it.

I would suggest that your gearbox needs to be checked. You can raise all 4 wheels (jack stands, or a hoist preferably) and put the car in second gear while the engine idles - obviously being careful not to touch any of the moving bits, but you'd be able to narrow down where the sound is coming from.

You can check the turbocharger yourself. Clean the area (cold engine first!) around the turbocharger, particularly around the air pipe connection to the front of the turbo. Then disconnect that air pipe and try turning and wiggling the turbocharger shaft. It should NOT wiggle, and it should turn rather smoothly (although when cold it will be a tiny bit reluctant to turn). Wobble in the turbo shaft does mean a problem and if you see this, the next thing to do is pull the intercooler out and closely examine the oil lying inside it for metallic particles (and then give the intercooler a clean).