At idle the engine bay is quiet. On the road, from part throttle upward, there is a whistle that rises with boost and disappears the moment the throttle closes. It is not there when the car is revved in neutral, which is the reason it has been so hard to find.
That pattern is close to diagnostic on its own. A noise that needs cylinder pressure or boost to appear is produced by something that only exists under those conditions, and in an engine bay that means a pressure difference across a gap that is otherwise closed.
A split in a charge hose, a joint that is seated but not sealed, or a gasket face that lifts slightly under pressure will pass air only when the difference is high enough to open it. The escaping air passes through a narrow, irregular slot, and a slot under pressure is a whistle.
The pitch rising with boost is the pressure difference rising. The noise stopping instantly on lift is the pressure being removed. Neither happens with a bearing, a belt or a pulley, because those depend on speed, and speed does not stop when you lift.
Start where the work was last done: couplers at the intercooler outlets, the throttle body hose, the joints around the charge pipe, and any hose disturbed during a recent job. A joint that is visibly seated can still leak at the seam where the hose meets the bead.
Then look at the parts that are hard to see: the underside of a hose near a heat source, the inside of a bend, and anywhere a hose passes close to a bracket. A split often starts on the side facing away from you and opens only when the hose is pressurised and straightens.
Turbo whistle follows shaft speed and is present at light throttle as soon as the turbo is spinning, rising and falling smoothly with revs. A leak whistle follows pressure and appears abruptly as boost builds. Rev the engine in neutral: if the whistle is absent, the turbo is not the source, because the turbo is spinning then too.
Belt noise is present at idle and changes with electrical load or steering input. A whistling pulley does not care about throttle. If the noise needs load and the neutral rev test is silent, work on the pressure side and leave the accessories alone.
Small bore lines are easy to overlook. A wastegate or bypass valve reference line that has rubbed through, or one pulled tight around a corner, will leak under boost and produce a whistle from a hose nobody thinks of as a charge hose. A leaking reference line also changes how the valve behaves, so odd boost behaviour can accompany the noise.
Trace each line by hand from end to end, looking for chafing, a hard kink, a split at a barb, or a tie wrap that has cut into the wall. Where a line runs close to something hot, the wall hardens and cracks rather than splitting cleanly, and that crack can be almost invisible until the line is flexed.
Blank one end of the tract, connect a regulated air supply to a port, and bring the system up slowly while listening and brushing soapy water on every joint. The whistle usually becomes a visible stream of bubbles at the exact spot, which is far more useful than listening in a noisy bay.
Do not over-pressurise, and do not pressurise through the turbo inlet without a blanking plate, because crankcase pressure can push oil past the seals. If the leak is at a coupler, the intercooler outlet bead and the hose bore are the two things to check; a silicone hose of the right bore, seated past the bead, seals far more reliably than one that has been forced onto an oversize pipe.
