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If the rear end gears were improprely adjusted you would hear it in the other transmission gears also, certainly prior to 30-40 mph.
Try this: Slowly AND without traffic around (find a smooth quiet road) slowly run up to 40 in 3rd let off on the throttle and listen for any unusual noises or rumbling, accelerate slowly in 3rd and do the same listening procedure (Be sure there is not traffic around you.) Then try it in 4th and then 5th allowing for a little higher speed but not to drowned it out by the exhaust. Sometimes if the rear gears are not adjusted you can actually feel them meshing improperly.
Good luck.........Be Safe
Rick
Rick
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Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way 
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