I have a '96 3.1L with 145k on it. It runs great and is getting about 27 per gallon highway. My only problem is that when I start it up it sounds like I have an exhaust leak coming from under the hood. Not a really loud noise like you would get from a muffler leak, but a low rumble kind of sound at idle and take off.. After it warms up, about 25 miles to work, it does not sound like it did when started, the sound is like a normal idle. I'm wondering what could cause this...possible crack somewhere that expands and contracts, ie:louder when cold quiet when hot. I have been under the car and checked the system, there are no loose pipes and I can not see any visible holes or cracks. Has anyone experienced this or can tell me what this might be. Or how I might be able to detect where it is coming from.

Thanks in advance for any thoughts or ideas.......

I have a similar sound on my 97 3.1. It sounds like it's coming from the rear manifold but I can't get a good look to see if it's a crack or what. A mechanic told me my intake manifold was leaking but I don't seem to have any symptoms of it like rough running or anything like that.

When you get leaks like that which go away as it warms up it's usually a cracked exhaust manifold...slightly loose mainfold to head mounting bolts will seal up as the engine warms too, but makes a sharper clicking type exhaust noise, not a rumble.

It's possible it's leaking at the connector between the pipe and manifold too...they can get loose or break a stud and act like that