7 1/4 Rear Differential ~ Experiences

Mine howls between 60 and 70 mph. Never beat on it, and the granny that owned the car before me didn't either, far as I can tell. Drained it and put in fresh lube - no difference. Has less than 80K on it - original miles slant 6 75 A body - big bolt pattern. Changed one outer axle bearing, seemed to quiet a little but never really went away. Must be the gears (?) Funky sealed bearing, round ball bearings, not the typical tapered roller bearing. I have an 8 1/4 waiting in the wings if/when it goes or I get around to it.