This is one of the strangest things I have ever seen. 98 Windstar, 78K,
running almost perfectly otherwise and no codes were found by AutoZone when I
had them scan it.
Every 20-30 seconds the gas cap makes a loud noise that closely resembles a
foghorn. It lasts from one to three seconds. I checked to see that the cap is
installed tightly. It doesn't seem to matter whether the temperature is warm
or cold, but it does it more often when the tank is less than 3/4 full and
sometimes not at all when the tank is full. It doesn't matter whether idling
in park, driving, or stopped at a light.
I'm thinking something in the emission system? But where would you look for