my 98 gmc jimmy with 4.3 v6 has started to have a rattle noise at start up, especially when its cold out. it sounds like a motorcycle chain being pulled across the edge of a table. it will do it as I pull away, but will stop after about a minute. its got 224k miles on it. should I be too worried? I am thinking about the seafoam engine cleaner. what could this be? thanks guys


just wanted to add this, I have had to start adding some water to the radiator now and then. not sure if its related though. it idles and starts up fine. oil gauge seems ok too.

 

My catalytic converter started making the same noise you describe. Next cold start crawl under there and feel the catalytic converter. Careful, it gets hot quick. You'll be able to feel the rattle if it's the problem.

Seriously investigate where your coolant is going. Check your oil dipstick often. Look for overfull or the level going up, even staying the same if you usually have to add some. If it's getting into the motor you can't find out soon enough. Welcome to the forum.

 

The catalytic converter on my Blazer has a loose heat shield and it does the same thing. At first I thought it was the transfer case. I crawled under after cold starting it and held the heat shield and the noise went away. The heat shield surrounds the cat and I haven't figured out a way to secure it yet. I'm waiting for 6" of snow to go away before I crawl under there again to fix it.

 

I would have someone hang out where you hear the noise coming from when you start it up.Sounds like it maybe a loose tail pipe or defective catalytic converter/muffler.