I'm not sure what the exact problem is, but it had something to do with vibration, and the fix has to do with dampening the vibration. There are a number of parts that needed to be added/replaced, which is why they needed the vehicle for awhile.
As for why some vehicles exhibit the problem and others don't, my personal theory is that some people notice it and some people don't. I've tried to show the problem to a number of people, and many of them can't identify any problem at all. I do know that it is more evident in the driver's seat...I think this is partly because the corresponding vibration can be felt through the steering wheel. Even on mine, it was more difficult to identify from the passenger seat.
I can still hear the noise if I am under power between about 70mph and 75mph, but I have to listen VERY closely. If I didn't know the noise ever existed, I doubt that I'd ever notice it.
Here is some of the pertinent information from my service invoice. The descriptions are truncated, but someone here can probably make out what they mean:
A STEERING VIBRATION & DRIVELINE VIBRATION/DRONE
CAUSE: INSTALL UPDATED PARTS FOR VIBRATION
ST3010 STEERING VIBRATION & DRIVELINE
VIBRATION/DRONE
1 41196-60010 DAMPER, DIFFERENTIAL
2 90080-10167 BOLT, WASHER BASED H
1 52380-60061 SUPPORT ASSY, DIFFER
1 51405-35101 COVER S/A, ENGINE UN
1 45219-60090 YOKE, STEERING SLIDI
1 45203-60120 SHAFT SUB-ASSY, STRG
FC: 44,99
PART#: 41196-60010
COUNT: 1
2727 DRONE NOISE, SERVICE BULLETIN ST3010