Dear Car Talk:

I have a 2008 Ford Explorer Sport Trac, which runs great. But after driving approximately 15 miles or so, it starts making a roaring sound. Sometimes the open-ended wrench symbol on the dashboard comes on. But as soon as I stop and restart the car, the symbol goes away.

I had the transmission rebuilt a little over two years ago for $3,500 at AutoNation where I bought the car. Any ideas? -- Willie



Yeah. I'd go back and reintroduce yourself to your friends at AutoNation. The open-ended wrench symbol is Ford's "drivetrain malfunction indicator." The drivetrain is pretty much the engine and transmission. So that doesn't narrow it down very much. But by scanning your car's computer, a mechanic can narrow it down a lot more.

Whenever the drivetrain malfunction light comes on, the car's computer will store a code that tells your mechanic why the light was triggered. From your description, my first guess is that your transmission is not upshifting for some reason. That could explain the "roar" you're hearing.

Let's say your truck normally shifts from second to third gear around 25 miles an hour. If it got stuck in second gear, by the time you reached 35 or 40 mph, the engine could be running at over 4,000 rpm. That would make it sound more like an Airbus A320 than a Ford Sport Trac.

If you're lucky, Willie, and you've led a good, clean life, it might be something simple like a sensor or a solenoid in the transmission. And if it consistently corrects itself when you restart the car, that does suggest something electronic rather than something mechanical. That would be good.

It could also be a sticky valve in the transmission, which would not be the end of the world -- or the end of the Sport Trac. And if you're really lucky, those guys at AutoNation who charged you $3,500 to rebuild your transmission will feel a bit guilty, take pity on you and fix it for you for nothing.

I wouldn't count on that. But it's worth going back and asking them what set the light off, and if it could be related to the work they did two years ago.