I would say wheel bearing would be a possibility, especially if it changes when you turn the wheel.
It may be your tyres too, agressive tread patterns can be noisy, or the tyres could be scalloped.
Diffs on STI's are pretty reliable, I mean, have a look at the size of them!
If its a diff, it will change if you get off the throttle, eg, it will sound different if your coasting or accelerating.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i