Installed a set of motive 3.70s in my 9-bolt they whined a little at first now it seems the howl has gotten louder, you can hear it most on decel at highway speeds. I know this is usually caused by too tight backlash, mine was .006" within spec although on the tight side. Also I slightly went over spec on my pinion preload (by 5 in.lbs) not sure if that would do anything. Checked the pinion to make sure it didn't loosen up its as tight as ever. So wondering if I should tear it down get a new crush washer and set the preload right and try to get the backlash higher .008"-.010" range (and hope it doesn't mess up my pattern) or do these motive gears just make noise no matter what? Or should I add some lucas or posi additive or 140 gear oil?

 

Gears, like anything else, "grow" when they heat up.

Personally I always set up rears on the loose-ish side for backlash, because of this. I'd rather have minor undetectable slop when th erear is cold, than the ring & pinion attempting to occupy the same volume when hot. 2 pieces of metal trying to occupy the same space = trouble.

See if you can add about .003 - .004" of backlash, and see if that helps.

ALso make sure your pinion preload hasn't changed. Try that first; if it hasn't, do the backlash thing.

 

Checked the pinion, it's as tight as ever. Only makes noise when you let off the gas, give any throttle at all and its gone. Only reason I didn't set the backlash looser was they only gave me .010" shims in the pack! So If I moved the carrier over .010" more that would have put the backlash too much

 

Search around for rear end whine here and other places and I think you'll find scenarios where no matter what they sometimes just whine a little even when setup correctly. Especially Richmond brand gears from what I've researched. Some guys have had a shop do the setup and it whines, then they have a different shop check it out and say it's perfect, and redo it anyways, still a whine. So you could chase your tail forever, or maybe slightly change your setup and it might disappear.