It sounds like a Skyline when you put racing exhaust on it. Gran Turismo is full of fail, if they would atleast use the z28 sound on the SS it would be acceptable.
I think a few others have this odd sound, it's with big bore v8's... I'm not sure what could have changed, perhaps "raspy" might be more actuate as its not a smooth deep roar, it's like two conflicting sound samples are running together... I don't recal others complaining about it though.
But had I not heard it in GT 5 I might dismiss it as one more lousy car sound...