I have a 1999 legacy gt with the 5mt transmission with about 200 thousand miles on it and whenever I engine braking (wheels driving engine) there is a very loud rattle coming from what sounds like the transmission. it really only happens at lower rpm (under 3000) and if I put the clutch in the sounds goes away. the problem also seems to get worse the longer you drive it and is very bad after long streches of highway travle meaning that more heat makes it worse. I know the center differential is bad so i've been assuming the the bearing on the diff was the ratteling noise but after doing some more research I'm begining to second guess myself. I've found a couple other posts online with the same problem but no one i've seen has found a solution. This is a video someone posted with the same issue. It's from a 2006 legacy but its the same sound so I leads me to beleve that its a problem with the 5mt transmission because thats really the only things that the cars have in commen. my best guess at this point would be that the center differential is just so worn that there is play in the spacing between the spyder gears. Any input would be much appreciated.