First post and hoping for some diagnostic help on a gear whine noise! 2003 suburban 1500 4wd. The noise sounds like a gearbox whine. It is not consistent with the engine rpm but steadily gets louder with increasing speed. It only makes the noise when in drive, putting the transmission in neutral when cruising totally stops the sound. I have removed the front cv shafts and front drive shaft so it's not the front diff. I am replacing the cv shafts so I have cut the shafts and left them bolted in. The sound does not follow gear shifting either, it follows the vehicle speed. I would think the rear diff but it's odd that the sound goes away when not in gear but still rolling. Any ideas?

 

Coasting doesn't load the gears or bearings like under power. Pull the cover on the rear dif. and look at oil color. Could be a pinion bearing if ring gear looks OK.

 

I pulled the cover. The fluid was virtually gone and burnt... Frustrating considering that I changed it last July. Well that narrows it down, thanks. Looks like I will need a rebuild.