my nephew has a 05 forestor, 42k miles, auto. apparently its been doing this for a month or more. just heard about it today and was asked since i have a WRX and work on that car. But i dont know about this problem.. when you make a turn, especially slowly it clunks. thats not the right word to describe the sound/feeling but its like the rear diff locks then releases. one wheel is turning more than the other and it sticks then releases and 'clunks' and it has a definite shudder/jerking feeling. if feels like the rear but i dunno... never had this experience before. he had a shop replace the rear diff oil which turned out to be clean in appearance and it made little or no difference. he also had it aligned and while they were doing it they said there was no issues with the axles, bearings etc. the car has zero mods. total stock. so.. is this a bad rear diff or center diff? what do you think. and if its the center diff in the auto, whats it cost (roughly) to have to repaired. im gonna guess if its the rear, best thing is to just get a replacement from a junk yard. guy at the shop (not subaru) had mentioned breaking it down and fixing it but i suspect the labor would be more than a junkyard replacement.
Really hard to say without hearing it. But I am quite sure Andy could diagnose it quite quickly. When it comes to transmissions, there really is not a better mechanic that I know of. I'm not sure where your nephew lives, but it's worth swinging by one day if he is anywhere close.