I was cruising in automatic Sport Plus mode yesterday and floored the pedal for 3 seconds in a very short tunnel. When I released I heard a loud explosion from the exhaust. Hadn't tried 'farting in a tunnel' before and don't plan to. After this I started paying attention to exhaust note with windows down and noticed something. 3rd gear @ 1500 RPM, floor the pedal to 3/4 and this is what happens:
Car starts accelerating with a low rumble and from about 2500 RPM I hear a rattling/tweeting air sound up until 4000 RPM, after that exhaust becomes too loud to hear anything else. It sounds like a
high-pitch wind rattle 'twe-twe'twe', not a typical exhaust air leak 'pr-pr-pr' 
The sound of turbo spool is different and has refined non-rattling 'swoosh' sound. Do other owners have this? What could it be?
Thanks for any advices