yes I have had that dump valve kind of noise. I know what you mean.
I was trying to figure out what it was' sounds cool though. its not loud no where near as a real dump valve on a turbo engine.
I always notice it in the evenings when the temp is cooler and the engine is hot.
it always happens when driving gently in mid rang up to 4k rpm using e shift controls.
not sure what is though -
off topic with noby76, talking about 530d, I am playing with E46 m3's getting infront always. we have an E90 330d m sport in the family and gave it a run. it cant stand well with the is300 in no angle. . IS300 is a undercover beast. not many know its capabilities. its a superb cruiser with hard reliability & the quietest engine I have owned with 215 bhp. and it certainly goes a lot more quicker than the book figures..
I never knew this until I owned one for 2 years.. for the value I would say the best motors out there. it certainly fixes up the big headed german owners!