Depends on who you talk to. A million theories about it and no conclusive answer from anyone. I got caught up in it when I started with 4G's and made sure I got a 6 bolt as most people reckon it was 7 bolts that suffered it the most, but now I really don't know.
Some say it's heavy clutches destroying the thrust bearing
Some say it's a small design change between 6 & 7 bolts in the thrust bearing area (fuck knows what that was)
Some say just change your oil proper and it wont happen
Some say it was a bad batch of factory bearings (thou plenty of people with rebuilt engines suffer from it)
Some say the stig makes it happen by using the force.
I don't know anything so I just make stuff up. Be interesting to see what the rattle is, cheap EVO if it is something simple.