2002 Windstar clicking on turning key

Hi,

No problems with a 2002 Windstar, 63,000 miles, until now. Turn the key, lights, stereo, engine diagnostics all working. Crank the engine. Get a rapid clicking noise but engine doesn't fire. Turn off key. Try again, same noise, but not as loud. Turn off key. Try third time, engine fires and runs fine. Clicking noise seems to come from top of engine area around carbuerator (definitely not in the area of the starter) Can you point me in the right direction? Thanks!