I have a 2003 Jetta and when I turn on the AC, it works (blows cold air) but a noise from under the hood-- a loud clicking or clapping noise happens. This started when I turned the car on one morning, the day after a really hot day with lots of city driving with AC on. I havent been using it since (dont want to make it worse) and would like some clue as to possible causes before I take it to a mechanic.