It's a fad to make them noisy... the ones I drive at work that are pounding out serious hp usually don't give you a clue unless your in the drivers seat, then you hear that burble which is really strange the first time you hear it. You generally don't hear it outside the car. The phhhsssst is by design... they even advertise how loud they are now. Easy enough to make it quiet... but it's a matter of what you want, presence or stealth? I'd say 95% of the kids adding turbos want presence, like when NOS went from a secret weapon for street racing to polished bottles lit up by neon and purge nozzles in the cowl with lights so everyone can see your 'power'. My mustand I used to street race was made to look damn near stock... well, polished out nice enough that it couldn't really be fast... could it? Stock looking show quality exterior and interior,hub caps,... and a custom 289 /built c-4/ and a selection of rear ends... with a huge nitrous system with a quiet exhaust system
Didn't make a bunch of noise while blowing the tires off at 80mph.
I'd also have loved to build the prostreet cartoon car... monster meats, huge engine, huge blower, graphics of stickers painted down the side, roll cage... but then who will race you? The father of one of my son's friends has a 514ci monster (600hp) with a custom Richmond 5 speed, and the monster tubs/tires, full Art Morrison chassis 67 Mustang... freaking awesome car, doesn't get driven much... it's a cop magnet and useless as an automobile, plus no one in their right mind would consider a heads up street race for money, it's so obviously a 9 second street legal car (and it is!) Cool, but not subtle and not a very good car for a daily driver.