Well, the engine isn't the only thing affecting the sound. There's the exhaust as well. STR might be using the same engine as Red Bull, but they might have different exhaust systems. This is why the Williams's sound differs from ther Mercedes sound.The STR does have a very strong whistling noise to it. I quite like it. But I find it odd that Red Bull, who are also using the Renault engine, doesn't have it at all. I don't think this reflects reality very well.