Jim, everything you've mentioned does respond to throttle and load. If you recall from 1955CJ-5's #5 post, " It does not seem to be a function of engine or transmission speed but it comes on noticeably at about 38 mph and starts to fade a little by about 45 mph...but it's still there." the sound is responding to speed. Also in the same post, he says running with out the rear drive shaft, the sound was not bad until a load was created with the e-brake. I also think this eliminates the e-brake as a source of the problem. 1955CJ-5, You said you do not know the history of the jeep prior to purchase and therefore don't know if a previous problem existed. To me, you did imply you were driving the jeep prior to the overdrive installation, post #12 "I would think the noise would change if it originated prior to the OD unit...which was new this spring". So I have to ask, was there a noise prior to the OD install? I think you are probably on the right track to suspect the aftermarket intermediate gear. I would also look closely at the shift fork that rides on the slider gear. Also, what weight gear oil are you using? An interesting problem that I hope we can help resolve.