5 gear is unique in that it's a 1:1 drive ratio, so load on parts is a bit different.
Have access to a dyno to see if the noise is present when the car itself isn't moving? It may be a bit scary to do it, but a probe pressed against the various underside parts while everything is wirling about on the dyno should find you a part that singing, if there is one (well, except for the drive shaft).
What happens to the noise on turns and lane changes?
-CTH
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