XC90 D5 Boost whine

[Enumero]

The car in question 2006 XC90 D5 Facelift, having covered as good as 300,000 miles. It’s wanted for nothing over its whole life, a folder bigger than my Mrs’ posterior full of history.

Over the last few months there has been a seriously annoying noise that appears with boost, but we (owner, myself, and each of the 2 garage it’s been into cannot find where it’s coming from.

It comes at around 1500 revs, but only when under load. Not when sat on the driveway. On the driveway it never comes no matter how hard it’s revved. The only way to induce it is to have your foot hard on the brake, and rev to 2000 when it comes again.

Smoke test has produced no results.

Replacement boost pipe at the rear of the engine (turbo exit to under engine solid section) - no change

Intercooler changed - no change

Turbo changed (removed and sent to east midland turbo’s for recon) - again no change 🤬

So today after the turbo was reinstalled and the noise still present, before reaching for the petrol can and matches I got the owner to do the brake pedal and Rev thing while I was over the engine, under the engine, all around the engine... the noise is definitely coming from the top nearside of the engine!!
It’s not a noise like a split boost pipe usually gives, though that’s not to say there isn’t a split somewhere...

Has anyone else ever suffered this?? Has anyone got any ideas on where else be looking?
The sound is like you’d expect from an air leaks from a solid section of pipe rather than any of the flexi pipes, it’s definitely coming from the turbo area. If it weren’t for the fact that I have been outside the car while the owner has been doing the brake and accelerator trick I would have thought it was a Caliper sticking and giving a whine to the brakes, that kind of sound.

Any questions welcome as we’re really starting to scratch our heads now...