My vote for the worst sounding car in the game is the Lancer 1600 Rally Car.
The reason for my vote is that it is your opponent on easy in the Special Conditions race at the Grand Canyon (depending on your car). And I hate those races with a passion and when you first encounter them - regardless of whether you eventually learn how to beat the races - they are painful - like one hundred thousand paper cuts. And that race in particular involves you constantly hearing that annoying buzzing sound behind you just on and on and on. You can't shake it and it just goes on and on and on. And if you make a mistake - as I invariably do at the Grand Canyon - it just goes by you buzz, buzz, buzz. On and on and on, but this time you're chasing it, watching it go round corners cleanly, buzz, buzz, buzz, on and on and on - never catching up, just getting more and more frustrated, buzz, buzz, buzz. Like a relentless swarm of honey bees, hunting you down to kill you.