My LR3 had an exhaust leak that sounded like that, quiet until I revved it up. Mine was actually leaking at the manifold to cat 3 bolt flange. I think it had shifted over time as there is no gasket between the two. Mine was consistent though or worse when cold and would get a little better as it warmed up because i think the exhaust pipe expanded which helped quiet it down.
Yeah, that's a common noise in some of the earlier Jeep 4.0s. IF it was this that would be great because it means there's not danger to the engine and it's not that hard to fix.
Update on my end. I got a mechanics stethoscope on Amazon. Pulled off all the plastic so that the throttle was exposed and sniffed/listend around.
The only thing that went in time with the engine rev and sounded closest to it was when I put the metal rod of the scope on the fuel line crossover that runs across the top of the engine.
I've never been more convinced that it's not something mechanical that's going to ruin the engine if it goes.
Right now my theory is that one or more of the injectors is extra noisy and may die soon. We are going to keep it and see how it goes.
That's not a terribly expensive job, is it? Replace all the injectors?
Al