Another update................
Just got the car back.
They said the pipes are fine and can't see a problem. They reckon that as I have no loss of power and it drives ok I should leave it and just listen out for it getting worse.
What do you guys think
That was THE answer I was afraid of... It either goes BOOOM after a while OR it just sounds funny and you drive happily ever after... As I'm having the same noise and no help whatsoever.
The most reasonable explanation was about "wastegate flutter" that is not harmful
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