The engine for the GT isn't officially confirmed yet, but it's rumored to be Mercedes' new 4.0-liter, twin-turbo V8 that is also going into the next C63 AMG. It's hard to make out any typical turbo whistle in the videos, but the engine definitely has a growl, whatever its layout is. It's just the kind of aggressive, angry sound a sports coupe like this should have.
The GT is confirmed to debut sometime this fall, but at the consistent rate of the released details, we should know a lot more about it by then. Scroll down to give a listen to the future Mercedes halo model throwing some revs and starting up.