Engine noise when atmospheric moisture goes up or when rains

I have a Ford Taurus 2001, 129K V6 engine. I have noticed that there will be a noise(as if some thing is rotating and hitting the engine ). When I got it checked, it was thought that a camshaft timer and base were the problem. After replacement, i did not observe any great change. This noise particularly occurs when there is a lot of moisture in the air or when it rains. It does not occur in extreme cold or extreme heat or normal weather. i feel it has to do something with moisture content. Can I get some insights into this.