1999 Ford Expedition 1999 Ford Expedition Stalls Immediatel

My Expedition began stalling during damp/rainy weather immediately after starting (2-3 seconds). I I changed the fuel filter, air cleaner and cleaned the MAF with MAF cleaner. The vehicle started but ran rough. I found a vacuum leak and replaced the affected section of hose. The vehicle starated and ran great but I could hear a "put-put" noise coming from around the 3rd cylinder on the passenger side. I can feel a tap when I place my finger on top of the coil pack for the 3rd cylinder on the passenger side. A mechanic told me the noise was a slight valve tap and not to be concerned. However, the sound does not increase in speed with the motor during revving/idling. The car ran for a week during dry weather but would not start recently during a rainy day. I am thinking it is either a faulty MAF or coil pack. Any ideas.