November 1990

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Dear Tom and Ray:
When I turn off the engine and start to walk away from my Nissan, I often hear a ticking noise. What exactly is causing the noise and is it anything to worry about?
Jack

RAY: What do you do for a living, Jack? I mean, if you're a foreign intelligence operative, then yes, a ticking noise coming from your car probably IS something to worry about.

TOM: But if you're not a foreign spy, or don't have an ex-wife seeking revenge, the ticking you hear is probably just the exhaust system cooling off. Your car has a heat shield, a thin metal covering that surrounds the exhaust system. Since it's thin, it cools off very quickly. And as it cools down, it contracts. If you don't believe things shrink when they cool off, reach into your swim trunks next time you go swimming in the ocean.

RAY: That ticking is the sound the heat shield contracting. It's the same sound the pipes in your house make when you turn the heat on or off.

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© 2006 by Tom and Ray Magliozzi and Doug Berman. Distributed by King Features Syndicate, Inc.

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Auto repairs can be costly! Save money by ordering Tom and Ray's pamphlet: "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.