Yea, agree with Alex, sounds like rear diff. They make speed and throttle dependant whines when they are on the way out. Eventually it will get worse to the point it starts clunking and lurching the car. When I pulled the diff from my old skyline I found one of the shafts that the spider gears sit on had sheared and was only being held (although very generally) in place by the other spider gears...
Mine started exactly like that, 70-80 was the start, and the loudest point as it got worse. The speed range it whined at steadily got wider and wider until it started at about 20 km, and started clunking at 100+, at which point I limped it to a mates place and changed the diff.
Took about 6 months to get to the proper f^*&ed point though, so you do have some time to change it out.
MY00 Red RX, a nice daily that was sold off once the B4 became reliable...
MY02 Blue B4, aka 'The Project' - v8 JDM STi engine package and assorted other 'goodies' - Sold, then bought back for parts. Stripped and crushed.
MY03 Blue RX, the newer old daily, pretty well more B4 than RX now... In the process of being converted to the track toy... Watch this space!
MY02 Silver Wagon, H6 conversion, full blue/black leather and almost full black interior to go with it.
MY03 White/Silver H6 Outback, donor car for silver waghoon.
MY97 Silver RX, acquired to become track car, but plans changed. To be stripped and scrapped in time.
