It's who sets them up and well as gear brand. I run Genuine Gear in my 8.8, were setup by a pro, and both sets are loud. The first set worse though. I've heard the same story from other guys running Genuine Gear junk in 8.8 as well.
I have a long drawn out theory about that dronning noise. you hear it when at speed and you slowly take your foot off the gas, right at the point where the gear transition between drve and coast. it got worse as I lifted my jeep and went away with the SYE so the finger points to driveline. but you say you don't feel any vibs... correct. because you have the pinion angle set correctly. those of you familiar with how a drive shaft accellerates and decellerates twice per revolution because of how ujoint caps travel in an eliptical pattern. this acelleration and deceleration of the driveshaft can get a little sloppy when there is no load present either power or coast. this causes the pinion to kinda like vibrate between the pinion teeth and ring gear teeth causing the noise.
again this is only my theory, but until i am proven wrong then i'm gonna stick with it. I have heard it both before and after a gearswap.
