when i asked Mike Shields about my rear diff noise, here's an explanation from his perspective. he mentioned i could post this for others to view. enjoy...
"The diff noise in the Subaru is quite common, as it was in my Datsun 510, which used the same differential. The noise is going to be especially noticeable with the bushing kit installed. The noise is mostly due to drive train ringing (rapid acceleration and deceleration of the drive shafts on over run) due to the relatively low inertia of the main drive shaft and drive axles. Other AWD manufactures will use more massive drive shafts, rubber couplings and other devices to eliminate this issue. Subaru engineering has opted to keep the inertia of the drive train low, which is one of the many reasons I drive an Impreza. This noise is generated at the ring and pinion (s) due to a designed amount of back lash between the ring and pinion. Miss the setup a couple of thousands of an inch and it can growl. My diff has sounded like it is falling out of the car for the past 100,000 miles and it had already 50,000 on it when I installed it. The other 4.11 WRX diff I have has 17,000 miles on it and I took it out because it was even noisier! I am sort of resolved to go into that diff to set it up a bit tighter in the hope it will get quieter."
"This brings up the second issue, which is the tolerance of the ring and pinion setup, which should never be too tight, rather a certain pattern is desired. This means there is a specific amount of free play between the ring and the pinion, a known backlash specification. I understand that there have been replacement differentials installed with an existing repair/exchange program by Subaru of America for those few units that are excessively noisy. For the most part, even though the unit is noisy it will run its normal life. Subaru just takes these units in and goes through the shim set up on the side bearings of the ring gear and the fore/aft placement of the pinion and then puts them on the shelf for exchange with dealers that have found unusually noisy units. There are also a pair of mass damper hockey puck type units that have been bolted to the "T" rear diff cross member (the forward one) towards the outer ends on the 98-01 RS and standard cars, that I have noticed is missing from the WRX. This was used as an attempt to dampen the noise from the diff as well. Many cars have used these types of mass damper for exhaust and sub frame noise and vibration damping over time."
"Since we are not the engineers, I have only speculated about this condition from my general knowledge of these differentials, having used them in my Datsun 510 and now my 93 Impreza for over 400,000 total miles. They will make noise a bit, especially if the STI hard bushing kit used, some units make a lot of noise and should be evaluated by the dealer if excessive."