98 Windstar gas cap making foghorn noise

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 the problem?
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I would change the cap, to start... and get the codes read out.
If it does it with a new cap, and there's no code regarding the recovery system and it runs fine... then forget it.
But I'm betting a new cap will resolve it....given everything else checking out okay.
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Thank you, I will check the carbon canister first to make sure it isn't clogged as suggested by Mars, and then proceed to replace the gas cap. This noise is very annoying when stopped in traffic.
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Pressure relief valve in the cap. Yours is probably blowing off too easily, here in NJ the state inspection actually pressure tests the caps and a lot of them fail. The more air in the tank the more to expand and "blow off" . That's why it does it less when full.

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I have a 98 Windstar too and this happened once before to me. The solution was a clogged Carbon canister. Have the dealer replace it. It cost about 35 to 40 dollars and the labour to change it.
It's not the Gas cap because its design to do that...suck air through a diaphragm. The Gas tank is plastic and it could implode the tank if the cap is replace with one without a diaphragm.
If you think otherwise...them the dealer will soak you for other parts that are more costly but not the problem. Bee them there, done that.
Hope it helps and let us know what happens.

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Thanks for the insight on this problem. I think it would be smart to check the canister first. Where is it located on the Windstar? I know the manufacturers like hiding them in strange locations. Can it be checked for flow by disconnecting lines and applying pressure or vacuum to the appropriate fitting on it? I really hesitate to take this in to the dealer as in the past they have insisted there are multiple parts needed to repair what appears to be a simple problem to the tune of several hundred dollars, and right now we simply can't afford it.
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