Dear Car Talk:

I took my car to our mechanic to have a clanking sound in the rear checked out. He resolved it by removing the heat shield by the muffler, which evidently had partially broken off. He said it shouldn't be a problem unless we "run it all day in a Kansas wheat field." How important is this heat shield for someone who does mostly city and suburban driving? Do you agree with his opinion, or should we have it replaced? -- Mike



I tend to disagree with your mechanic, Mike. The heat shield is there to keep your 400-degrees-F muffler from setting fire to the wheat field you're parking in. It's also there to keep it from setting fire to the contents of your trunk.

It wraps around the muffler and, depending on the car, the muffler can be pretty close to the underside of the trunk. We've seen instances where people have come in complaining of a burning smell, and we've found the underside of the carpet in their trunk melted. We haven't found a set of golf clubs fused together yet, but we have seen evidence of significant heat.

Now, maybe your car is one of those where the muffler isn't right up against the underside of the car. But we don't know. And here's the other thing to consider: The heat shield -- on most cars -- is welded to the muffler. And when the heat shield starts to disintegrate to the point that the noise is annoying you, the demise of the muffler is not far behind.

So my recommendation would be to go ahead and replace the muffler. It'll come with a new heat shield, and you'll probably need it soon anyway.

I mean, if you drive the car only three miles a day, and the exhaust system never gets really hot, you probably can get away with not having a heat shield. There are plenty of people who do.

But if the circumstances all are against you -- your muffler is close to the undercarriage, you drive a lot, and you sell lithium-ion batteries door to door and carry them in your trunk -- we might not hear from you again.

So a new muffler and heat shield is the safe way to go, Mike.