Centurion Nebulus Nimbus in Astérix and the Big Fight. Only time he speaks at a normal volume is an attempt at whispering. The rest of the time, HE IS VERY LOUD. In the original French, his name is Langélus, a type of church bell. And church bells need to be...? That's right, LOUD.
Siren was raised in the Sonic Canyons, where constant ear-splitting noise is a part of everyday life. He's probably never going to adjust properly, all told. Also, he's a police car; do you really want him to be quiet? Alternatively, he may have come up in a raucous, industrial area of Cybertron full of rough-and-tumble bots, where he learned being anything but loud and obnoxious meant you were overlooked or picked on. There's conflicting canon on the matter.