Have you crawled under and investigated stuff?
Is this a Gear Grinding sound or just a "Racket" that lasts for a second or two?
When you are under there, look for anything that can move/ twist/ or torque when the engine torques.
Once on my CJ5, The bolts that held the tranny/ tranfer/ rear of engine to the skid plate, fell out. All worked fine until I put it into 4X4, then it ground to no end. When the engine torqued, it would slide everything across the skid plate, to the passenger side. The Yoke on the front drive shaft would hit the frame while it rotated. In 2WD it was not turning so I didn't hear it, I just sorta missed that it was sliding around, for a while--Who'd-a-ever thought that?
Just to maybe help start you out, this is the "U" Joints under my 1946 Willys
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Images (click to enlarge)
Thursday, April 21st, 2011 AT 3:43 AM