I've got a similar problem with my 90, It's fine up to 50mph then the noise starts to get louder and louder (4th and 5th gear only), very similar noise to what you're talking about. Doing some testing this morning it disappears when I take my foot of the accellerator or dip the clutch. Could it be the release bearing on mine too then? Am I guessing correctly that it'll be a bugger to change?