It seems very quiet for such a powerful vehicle.
Porsche continues to develop its many variants of the upcoming 911 Turbo, and this spy video catches the convertible version lapping the Nürburgring. This one is a little odd, though. It's not clear whether the driver just isn't pushing very hard or if the exhaust tuning keeps the engine very quiet. Regardless of the reason, the powerplant is generally hard to hear over the tyre noise from this test prototype.
Porsche generally prefers to use black test prototypes and then apply matching concealment panels to disguise the body. Looking closely at this one, there is a piece over the front fascia to hide the final design. At the back, the Turbo's broad fender wings gain inlets that aren't on the standard 911 Carrera. The rear end wears an active spoiler that you can see at work in this video. There's another covering over back bumper, but the quad exhausts are likely the production-spec parts.
The new 911 Turbo's powertrain is still a mystery, but rumours suggest it uses an updated version of the existing twin-turbo 3.8-litre flat-six engine. In standard trim, this improved powerplant allegedly makes around 600 bhp. The more potent S version allegedly pushes the output to 640 bhp. Both versions would reportedly run through an eight-speed dual-clutch gearbox to an all-wheel-drive system.
The 911 Turbo should debut before the end of the year, possibly even having a public display at the Frankfurt Motor Show in September. The convertible probably wouldn't premiere at the same time, though. Look for the droptop to arrive a few months later but potentially still before the end of 2019.