In order to diagnose this I have had to raise the driven wheels off the ground, disengage the traction control, and put the transmission in drive, then observe what is happening. Of course, you have to take the necessary precautions to make sure the car cannot come off the supports or roll in any way; you must have level ground and the remaining wheels chocked if it's two wheel drive, which I don't think is the case with your Subaru.
Do not exceed 35MPH with the drive wheel tires unloaded, the centripetal acceleration without the load could cause a tire/wheel assembly failure -- and it could be pretty catastrophic, too. If you aren't 100% certain you can do this safely, have a professional shop check it out; they'll put the car on a lift.
Use a mechanic's stethoscope to carefully probe each component and find the source of the noise. If it is safe to do so, you might find a bad wheel bearing by touching the coil spring with your finger, but DO NOT touch the spinning wheel, and do not attempt doing this unless you have at least several inches of clearance.
I have also had some limited success with a set of "chassis ears" in finding problems like this. Google for details. It's a PITA and the tool is not inexpensive, but sometimes it's the only way to find something on an AWD vehicle when you don't have a lift.
