I can't believe I found this thread. I have the exact same noise, completely identical. I thought I was going insane (noises are tricky to google for). Mine's a 2001 4.0 5-spd NV3550 92k Miles. Mine is loudest around the 1900-2000 rpm mark, almost imperceptible on cold start but once things are warmed up after probably 2-3 minutes I can hear it annoyingly clear, most easily from inside the cab as others have also stated. Clutch in or out, in gear or neutral makes no difference. I haven't tried rolling in gear with the engine off so I'll give that a shot and see if I can verify I still hear it like others have.
I did find an exhaust leak where the two pre-cats connect to the manifold and got the bolts tightened up and sealed them with a thin layer of high temp gasket maker, did the vacuum and soap trick and can't see anymore bubbles but it made no difference to the sound. I originally found the leak when slowly pouring seafoam into the intake while running. This all started for me assuming it was lifters/injectors so I put 6oz in the crankcase and the rest when in the intake (This has quieted up the overall engine noise/rattle). Let it sit for 10 minutes and turning it back on and saw a decent amount of smoke coming from around the manifold area, I kinda wanna try again just to see if all the smoke really was coming from the pre-cat/manifold connection or to find a crack. Smoke is much easier to see and it's only $7 a can.
I would like to point out that I didn't notice this noise until relatively recently, after my most recent oil change in July. It has been steadily getting louder. Actually now that I'm thinking about it the whole motor became quite a bit noisier since then. I'm curious to know what oil you guys are running? My last change I put in 6-quarts of synthetic 10W-30 from Costco, only difference from what I would usually use is brand because it was a good deal. I might go to dino oil next and see if that shuts things up at all.
Another thing I'll add is I'm seriously under-geared for my diff/tires, 32 inch BFG and 3.07 gears (Shoulda just got stock tires lol). I really don't think this can have anything to do with it other than possibly wear at some point in the drivetrain from stress? I'm usually driving in the lower RPM range due to this, probably lugging the engine. The only reason I even mention it is becuase I'd just like to throw out as much information as possible so hopefully all of us can find the common thread that ties us all together.