It kinda reminds of a turbo... the noise i mean. It happens at 2500+ rpm. Im trying to fix as much on my XJ as possible. So while I was under my xj looking at the steering to make sure everything was tight (I have high speed death wobble... Still trying to figure that one out) I some thing connected to the transmission and it kinda looks like it need a hose connected to it. (will post pic) Could that be what makes the wierd noise? I checked the fluid and it seems to be ok. If not I have more form my Tcase swap.

Any other reasons why itd make a weird noise when its at high(ish) rpms?

The second picture was the best I could get for some reason

 

Do you still have the stock exhaust? When we built mine, i forgot to put in a gasket at the down pipe, and it whistles like a turbo from 2300 rpms on to 3500 or so.