The Citroen GT is by far and away the loudest road car I've ever heard. Admittedly I was sat on the ground at the back of the car, but even just idling it sounded immense, like a petrol LMP car.
Loudest sound from any car? Race of Champions, Stade de France, 2006. Two jet dragsters did laps at walking pace inside the stadium, firing the afterburners every 10 seconds or so. It was almost so loud it was briefly silent, and you felt it more than anything else.
The first time I heard an F1 car start up created an instant memory too; I was 12 or so, at Thruxton, standing behind a 1993 Footwork. It made quite an impression.