I bought this car last fall, and this noise has been present since I got it. Thinking it would get worse (and easier to diagnose) I waited a while before going back to Subaru. I bought the car with a Subaru used car warranty. It's an '07 Spec B. The warranty expires in Oct 2012, so in I go yesterday. I explain this oscilatting noise/vibration occurs only between 50-55mph, isn't felt in the steering wheel, but if you rest a foot on the clutch or brake pedal, its easily felt. Any other speeds, the car feels great. I tell them the noise did not change when I had my snows on during winter, snows on dedicated Amp rims. The wheel bearing swerve test has no impact on the noise, however accelerating makes it stop, coasting from 55 to 50 makes it a little louder. Pushing the clutch in has no effect, the gear has no effect either.
Their diagnosis: they tell me my tires are the problem. I reiterate that the noise/vibration was present with other rims/tires on the car, they maintain they can't do anything until new tires are installed.
Today I install new tires (mine were almost worn out anyway), no change. So, I write here hoping someone has experienced the same thing and can detail what was done to correct the problem.