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I've owned 4 turbo's, and driven a good number of others; stock and moddified. The GNX and T-Type come to mind for vintage turbo setups.
The loudest and coolest turbo sound was the Laser/Eclipse, 1990 vintage.
Plus, it had a true blow-off valve. That was a sweet sounding turbo.
The '81 Audi 5000 turbo was next loudest, oddly, but it had very limited boost, so only at high speed and high rpm, mash the throttle, downshift and nice.
Between the A4 2.0T and my 135i, they are both pretty quiet turbo's. Obviously tuned for a different market. The A4 was a bit louder and got a bit louder still after the APR tune upgrade. My 135i is actually disappointingly quiet for a twin turbo setup. Plus, both the A4 and 135i systems don't use a blow-off, but a recirc valve, so in between shifts they are more efficient by helping to keep boost up, but the wonderful turbo sound is nearly non existent.
Those of you that hear it must have your windows down and no audio playing, and are going pretty slow, cause the wind noise would easily over power the turbo sounds in the 135i.
The previous gen WRX and EVO also had great sounding turbo systems. The new WRX, after the rework, sounds decent too.
BMW seals the engine bays pretty good, but at least you still hear the wonderful sound of the inline 6 when screaming past 5k. To me, that's the coolest sound of this engine.
In my Laser I could hear the turbos even with the stereo playing quietly.
I wonder if the N55 will have a better turbo sound, as it's using a larger single turbo.
I bet it'll still be relatively quiet, again due to the way BMW seals things, which is quite good.
As one poster said, I've got to be in a tunnel or underpass with the window down, not going too fast, and the stereo off. Then, with a double gear downshift and WFO I can hear the faint and sweet sound of those little turbo's whining.
OP, if the sound you hear is pretty loud, then either you're hearing is really good and you are just hearing the turbo's, or you may be hearing a bearing, a big intake leak, a leak at the intercooler, a belt (explaining why turning the AC off can help), or some other oddity.
I suggest you go to the dealer or find another 3.0TT driver, and take test drive.
If the other car sounds the same, then it's likely your turbo's, then be happy your hearing is that accute and you can enjoy what BMW has tuned out for most of us.
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