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'98 Ford Explorer engine sound

I bought this SUV 4.0, v6 2 door automatic in '03 with 37,000 mi. Installed a tranny cooler & weight distribution hitch and pulled a 3000-3500 trailer.

After a month or so of traveling I began to get this sound like a fog horn or loud humm apparently from the engine compartment at the end of the day when I was manuvering into a campsite. There was no indication of over heating, gauge changes, no check engine light, performance change etc. Also, shortly after unhooking, I could start it up again & no sound.

Year after year the sound started earlier and earlier in the day and then it started without pulling the trailer. A few years ago it became constant a little while after warm up now with 111,000 mi. I frequently check filters & fluids.

This only occurs in warm weather. I was in Ohio this spring & it was chilly and no sound. As I headed south and warmer weather it became more frequent. Now that we are home in Florida it is back to constant a while after warm up.

I have had this to several garages and have spoken to several Ford service managers and no one has a clue. Hopefully you can help.

Thank you,

Ed Mosshammer

I would check the entire exhaust system to see if there is a place where the a component is touching the body, frame, or hitch attachment. You have to visualize that the entire system is going to grow in length as it heats up. You might put it on a lift when the noise is present and see if the ‘grounding’ point can be seen. I suspect a readjustment of the hangers will probably solve this problem.

This sound is generally associated with the IAC valve. A pretty common complaint and any Ford tech should know it. Can sometimes be spray cleaned. I found that it can also be caused by the black plastic filter on the side getting clogged. Best to just buy a new one for $65.

Second vote for the IAC. Failing ones are often described as having a “fog horn” sound.