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We have a 2001 Mercury Grand Marquis that developed a rattle over the last week or so. It rattles on both sides of the car when the car is in neutral, and disappears when the car is in gear. Any suggestions?

Rattles are very very difficult to diagnose…ESPECIALLY over the internet without actually hearing the noise.

The only way is with the car running and in neutral…try to pinpoint the location of the rattle(s).

Before you do that you might want to clean out your car including the glovebox. A while back a customer came in for a rattle that he could not find. I spent 15 seconds in the car and pulled out 5 lugnuts from the glove box… problem solved.

I was wrong. The rattle occurs when the car is in drive and at a complete stop. When it is put into neutral or park, the rattles disappear. If the car is in motion, there is no rattle. We hear it on both sides, and I want to say that it originates from the bottom of the car (underside). Would that give any more clarification?

Now that you have clarified the situation, I am pretty sure that you simply have a loose or bent heat shield. These thin pieces of sheet metal (which are there to protect other components from the heat of the exhaust system) are fairly easy to damage if you run over something on the road.

The good news is that a bad heat shield is not dangerous. Just have it attended to at your next scheduled maintenance. And, the mechanic will probably charge you nothing, or perhaps only a few bucks, to take care of this problem.

A friend looked under the car and he said that the heat shield seems to be fine, and he suggested that it might be a transmission problem. If we assume that it is the transmission, can you give me a guesstimate as to what we are looking at?

A heat shield can look fine but still rattle at certain engine speeds. It has to be physically checked. Put the parking brake on, put your foot firmly on the regular brake pedal, and put the car in gear. Have someone give a listen to try to pinpoint the source. If it is coming from an area where a heat shield is located, have the person push on the heat shield with a broom handle and see if the noise changes.

When you put some load on the engine with the brakes on, the power train will start vibrating, and the exhaust system will likely move enough to bump into something. This is very common. Have someone put the car in gear with the brakes on and stick your head under the car, or take it to a shop and put it on the hoist with someone holding the brake. Muffler shopa do this all the time.

the heat shields are pesky in that they look fine, but often they get s small pebble inside (between the shield and the converter) and these pebbles sound like a large rattle. i have had them in my van shield, and the only way i got them out was to cut a 1/2" hole in the bottom of the shield so they could fall out!

so you looked and saw they looked alright, but you need to inspect them closely, not from the side of the car, but underneath.