Audi A4- Driving 50 mph and engine quit. It doesn't crank over, just makes
a slight whirring sound when I turn the key????? Any thoughts? Thanks.

 

Probably have a broken timing belt. How many miles does the car have? What size engine? Have you ever replaced the timing belt? If it is the timing belt, you more than likely have bent valves, and have a very expensive repair on your hands.