Drivetrain ringing

[ApexRK]

The car: 97 Kouki, S14 SR with auto to manual swap done by previous owner, stock clutch, new tranny fluid.

Car has always made this drivetrain ringing sound as long as we have owned the car and I have no idea what it could be other than knowing its something related to the drivetrain.
It only occurs when decelerating or with a constant no load or low load situation, once power is applied its gone or if the revs are below 2500rpm the sound is gone. Also if I pop it into neutral or put the clutch in the noise is gone. I know its not a backlash related sound.

The Sound: hard to describe, but if you have ever had a metal shift knob loose and makes a loud ringing/vibrating sound as the threads vibrate thats what it sounds like and its coming from outside the car. My first thought was its something loose in the shifter, but i checked it and its not. The ringing sounds like a fast ringing, not like a school bell, quicker than that.

I plan to jack up the rear wheels tonight if I have the time and run the car into 2nd gear to try to create the sound, but before I do that has anyone encountered a sound like this and maybe can give me a heads up on what to look for?