Rented a 4GTS from Porsche Drive this summer in Stuttgart. Loved it, what a monster. At 150mph, I wasn't really noticing the noise, just trying to have fun. When we would go through small towns through the countryside, I would turn off the PSE to kill the popping. All the noises it made were good Porsche noises. Nothing objectionable at all.

 

My wife's AMG C63 with the 6.2l V8 made way more noise starting up in the garage than my C4 GTS. When I'm driving, the C4, the major noise inside the car is tire roar. You can barely hear the exhaust compared to the tires.

 

My Carrera S with PSE has a an awesome growl for about the first 30 seconds after a cold start. I've read on the forum that the car starts with the exhaust valves open for emissions reasons. While not the loudest of cars, it will certainly get people's attention on a quiet morning. I am mindful of this as I know my neighbors can hear it. When I pull into the garage, my family reports that the car makes the house resonate in a noticeable way. Enjoy your GTS. I'm sure it will be an awesome ride!

In the meantime, here's a cold start to get you started!