Not bad [:
Like you said, the engine sound did get a little annoying by the end. The only reason I can think the audio might be doing that is because instead of streaming it you've used the "Start" & "Stop" method. Sometimes this creates a popping sound when it loops or from a sudden stop. It's also more likely to happen if you've extremely diminished the quality of the original audio.
As for game play, some music would have been nice. The NG Audio portal supplies free tracks, some of which are high quality ;)
It wasn't extremely fun, but it held my attention. After about 30 seconds of driving, I felt like ramming cars for fun. Which makes me kind of crave a mounted turret on my car, but you know, whatever [: