Driving with rocks in the engine? Sounds bad

Dear Car Talk

Dear Car Talk | Apr 01, 1991

Dear Tom and Ray:

I am the owner of a 1984 Thunderbird which has one slight peculiarity. When it's started and the engine is cold, the engine is relatively smooth and quiet. But when put in drive (when cold), it sounds like it's grinding rocks. I've had the transmission checked, the engine mounts checked, and I'm constantly on the lookout for metal shavings or other signs of material destruction. I would greatly appreciate any thoughts or suggestions you may have regarding this situation.

RAY: Well, Daniel, my guess is that it's nothing serious. When you put the car in Drive, you put a "load" on the engine and lower the idle speed. That makes the engine run less smoothly and increases the tendency for the whole car to shake.

TOM: And the things most likely to make noise when they shake are pieces of the exhaust system. I'd look first at the heat shield, which is a thin metal covering around the catalytic converter. I'd also check the rest of the exhaust system to see if anything is touching the underside of the car. If it is, a minor adjustment should make the noise go away.

RAY: If the source of the noise remains elusive, ask your mechanic to check the catalytic converter itself. Inside the converter, there's a honeycomb that can vibrate and make a grinding sound when it's starting to disintegrate. It sounds sort of like the Tin Man from the Wizard of Oz gargling with pebbles.

TOM: You'd be most likely to hear the honeycomb vibrating when the car is cold (remember, things shrink when they get cold) and the honeycomb has contracted. When it's smaller, the honeycomb has more room to jiggle around inside the converter. When the car heats up, the honeycomb expands, and the grinding noise diminishes and can even go away. But as the disintegration gets worse, the noise will be there more and more of the time. That's actually good, because it will help your mechanic find it.

RAY: If none of this advice helps, Daniel, you'll have to resort to your "fall back" position: When you're driving along, and the rattling part finally FALLS off, go BACK and take a picture of it. Then send it to us, and if we can identify it, we'll tell you what it is.

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