The boost control valve of the turbos, the wastegate, wears out, and the rattle can come from two places: either the flapper knocks against the sealing face, or the shaft is loose in its bushing. The noise is most audible at idle and at light load.

As the wear grows the assembly gets looser and looser, and the valve either does not close properly at spool-up or sticks open. Boost then falls away, the engine loses power and can go into limp mode. In community reports the 30FF underboost code goes with this.

The repair is not necessarily a turbo replacement, and that is the point. A repair kit is available, for both turbos at once: a stainless bushing, flapper, wastegate arm, rod end and retaining clip. The diameter of the bushing goes up from 11.5 millimetres to 12.6. The kit costs 95 dollars, but the turbos have to come off, the new assemblies have to be pressed into the turbo housing and then welded. That is work for a specialist, but it can be done in a day.

One thing is worth knowing at a viewing: adjusting the nut of the actuator arm can reduce the rattle temporarily. A quiet engine on its own therefore proves nothing.