The HO 72 is a great, rugged rear end, but the ones I've dealt with all seemed to have a little gear whine. There is an adjustable side thrust bearing on most of the ones I've seen, with a brass disc that bears against the side of the ring gear with an adjusting screw and a locknut on the driver's side of the carrier housing. It's there to prevent the ring gear from deflecting when pulling a heavy load. To minimize ring gear flex under load, loosen the locknut and very gently turn the adjuster clockwise until you can feel the thrust button contact the ring gear, then back off ever so slightly so it's not rubbing the gear all the time. Tighten down the locknut and you're good to go!
Jerry
