Engine/Exhaust? Noise @ Start-up
 

Whenever I start my '97 F-150 5.4 on a completely cold engine, it makes a pretty loud rattling noise from somewhere under the hood. It only lasts for about 2 seconds, but I can't figure it out. It's pretty "tinny" sounding, so maybe something in the exhaust? This seems to get worse the colder it gets. It doesn't make the noise at all on a warm engine startup. Thanks for any help or ideas.

You might want to check and see if you have a shield that covers the muffler. This is typically found on the rigs equiped with the off road package. It's common for a weld to come lose on it and make a rattling sound. It did this on my Navigator and would usually do it on a cold start up. You can either have a muffler shop weld it back on, or remove it. I chose to remove mine, as I wasn't ever going to go off roading with my Navi.

It doesn't really sound like a loose shield, especially since it doesn't ever make the noise except for a second or two on a cold engine start (seems to only happen as the engine "spins-up" to initial idle RPM). It's a difficult sound to describe, but it's definitely not a buzzing noise, more like one or two pebbles being twirled around in a tin can.

Mine was very similar to yours. It drove me crazy for a few days. Mine would usually only rattle at initial startup also. It's worth checking and at least you can eliminate it as a possibility.

I hate to be even the suggester of bad news, but I had the same issue with my old vehicle, and it turned out that my cats had started to go bad. When their cool, you'll have some loose platinum media, but as you warm up (quite rapidly, I might add) the loos bits will fuse to the inside of the cat. If you're running ok, they've just started to go, but soon enough...

Go to midas or anywhere else and get a heat inspection. They'll measure the time it takes for your cats to reach operating temp. Also, when cold, try starting it up and then banging on the cats with a rubber mallet- if they're going, you'll hear it.

best of luck

noel