Could be the way or where you hear the sound. Check for an exhaust leak at the headpipe to the manifold. It will be minor if that is it and you will find it by let it run and crawl uder and see if anything is blowing out. The sound will vary with RPM and show up to your ear at the same rpm all the time.
You could have something underneath rattling at that rpm so while underneath check all the exhaust system to include the cat heat shield.
If not exhaust check driveline u-joints for security and installation maybe a strap is loose and at that rpm it vibrates.
Best I got.
89YJ 2.5L TBI, AX-5, NP231 SYE, Tom Woods Shaft, 33" BFG T/As, 4" lift.
AF Jeep Club Tail #184