Aaa, not for diesel then? Why not? I know the throttle is always open but if the valves are close the pressurised air as to go somewhere?
The sound is a funny one one, hard to tell, wwoooshh, a bit like something sucking in too, when the waste gate open I guess. I thought it might be the turbo lowering its speed with the air going backward like explained on wiki...
Cheers.

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E300TD year 2000. RUSTY SOLD
cost a fortune to maintain on the road
but run well on WVO
Second Merc died due to corrosion ( NOT rust) How can mercedes get away with that for so long?
Third lasted a month then went away...
Fourth now... Corroded too...