Learn to mimic the sound and then time it, when you close the door, let 'er rip. I do that for my unprestigious sounding engine....so when people get in, I let out a nice throaty roar whenever I accelerate, works a treat and doesn't cost anything. Try it out before settling on the sound, I'd go with Komisiripala's 'Clunk', first, then maybe a 'Boomf' next.