For last 2 weeks, in the morning, when I start my car, I hear a metallic rattling noise. It is only heard when I apply breaks; when I accelerate the noise goes away. After 2-3 minutes of driving, I no longer hear the noise when I apply breaks to stop at intersections, etc. Yesterday, I opened my driver's side window, and noticed that the noise was much louder and apparent to those who might be outside the vehicle than to who is sitting inside. Any ideas on what might be going on?

 

It's quite possibly a loose heat shield. Check the bolts on the header heat shield. If its not that, there are a few underneath the car. What kind of car? Year and trim level?

 

1997 Honda Civic. 150,000 miles.

 

Wouldn't a rattle from a loose heat shield be heard when the car is moving? I hear the noice when I'm braking. And why do you think it would go away a few minutes after driving around for a while.

 

At 150K, it's very possible something has rattled loose. If it's a DX or LX, there's the heat shield on the header and then two heat shields on the catalytic converter, which is attached to the header on the front of the engine. If it's an EX, the catalytic converter heat shields are under the car in the middle, a bit behind the wheels.

In order to pinpoint the source, I'd have a friend get in the car, put it in gear, then hold the brakes. I believe the reason you only hear it at certain times is because of the resonance frequency of whatever is rattling. When you have the brakes pressed down, there is a load on the engine, making it idle lower and closer to the resonance frequency of the metal which causes the rattle.

See if you can find a more localized source of noise. We can definitely help you.

 

Wow, thanks for the info man. I'm in med school, and this is sort of like finding out what's wrong with someone. I'll check in the AM tomorrow and post the results.

 

You'll find that diagnosing car issues is very much like diagnosing patients. Posting on a forum is like getting a differential diagnosis. You check symptoms and pinpoint sources, compare results to known issues, treat sometimes through trial and error. There are initial diagnoses that can be right or wrong, primary and secondary "infections," and the fun parts where you confirm a diagnosis and fix it.

 

I did what you recommended. I saw for myself what was vibrating to cause the rattle. I don't know what it's called, but I don't think it has anything to do with the heat shield. Attached is a picture of a metal ring around a pipe at the midpoint under the car; it was vibrates/moves a bit and causes the sound. I can't quite get to it without lifting the car. I don't want to spend a whole bunch of money and get ripped off by the auto repair folks just to fix it. Any ideas? Maybe have them tighten it the next time I get an oil change or just live with it? Is it crucial it be tightened?

 

I'm not 100% sure what that is either. Looks like a ring holding some heat resistant material to the pipe. It probably helps keep too much exhaust heat from warming the body of the car. From that pic, I can't tell if its broken or loose. You could ask a shop to look at it. They might not charge you anything to tighten it, but be sure to ask them about it before you let them do anything.

 

Nice...case closed I guess. Thanks for the help.