Chrysler PT Cruiser

I have owned my 2004 PT cruiser (automatic transmission, 4 cylinder 4 door) for about 4 months. I have noticed recently that I hear a ticking sound when my windows are down. It almost sounds like locus, but it is not constant. It often increases with acceleration and today, when I was at a stop light, it was quite loud. As I accelerated it lessened and then got loud again. I don't have any other issues that I am aware of to narrow down this strange sound.
July 9, 2007.

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