This is something my Contour does too. Occasionally under hard acceleration loads, especially if I jab the throttle, I'll get a little low clattering noise, it sounds like the valves are slapping or something. Its a very curious noise. I wouldn't worry about it though. When I was in California this summer, we had a Crown Vic that had the same problem, and it manifested itself every time we left the hotel parking garage. The car itself was really low miles, and even though it was a rental, it was very new, so I don't think its a maintenance problem.
