My bet on the noise would be the gears. As you start adding the steeper gears, along with suspension mods, you're sacrificing some of the OEM noise, vibration and harshness...in essence you're sacrificing creature comfort for performance. The control arms with stiffer bushings will transfer more noise than the driveshaft, as a matter of fact, the damping provided by the AL driveshaft is greater than than provided by the OEM steel one. If you want the 'race car', you have to sacrifice a little on the NVH, especially the noise. The interior begins to act like a big acoustic guitar, just amplifying all the road/driveline noises. Take your back seat out and you'll really see what I mean.
Everything may 'look' fine in the rear but all's it takes for a little noise is too much/little bearing preload, gear runout, pinion depth, backlash, etc. Does the howl change from drive to coast? If so then I'd definitely be looking at the gears.
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Jeff Chambers
Trophy Stock #3
11.462 Seconds / 117.43 MPH
Chambers Racing Team