Small noises are irritating Blain but if it's speed dependent, I'd have a go at Stapos route, wheel balancing first. Probably the cheapest of the options and if your local wheel place has a friendly as well as competent service, they will tell you how much and which wheels are out. It's very easy on our poorly maintained roads (that was polite, eh?) to throw a wheel weight because of a pothole, and sometimes they just fall off anyway.
Wheel balance issues usually show up at higher speeds though. I have a slight turn in one of my rims and I used to get a bit of wobble through roundabouts because it was a front wheel but otherwise the handling was ok until the wheels were rotated during a service and the wobble disappeared but, shortly afterwards I started having some issues with the rear end. Tommys suggestion is good too and if you are going to rotate the wheels, go out on a regular route involving straights, corners, roundabouts etc and note how it behaves. Then rotate the wheels and go and do the same route again at the same speeds. If your rim is the problem, the front should now be good and perhaps, and I mean only perhaps, you might notice something from the back end.
There is one other one that hasn't been mentioned here, diff carrier carrier bushings. These are a known problem/weak point on the Imprezas and Foresters. Because of the all wheel drive, sometimes it's difficult to isolate a knock or vibration due to everything being transmitted through the whole 'H' of the shafts and diffs, you might hear it as being in the front but a back seat passenger might say it's in the back. One of the other symptoms of the diff carrier bushings being shot is a knocking/vibrating sound under braking on uneven surfaces and rear end sway if you boot it and then pull out to overtake as opposed to pulling out and then flooring it. Does the Leggy do the vibration thing at the corresponding revs in 4th? What about under throttle, even gentle, at these revs in 4th and 5th? If yes, then the diff carrier bushings or any of the transmission mounts.
Sticky caliper as Goose said, but going up to 4 pots is costly, especially if that isn't the problem but it is easy enough to diagnose. Drive for a few minutes without using the brakes as much as you can (dual carriageway/mway) and coast to a stop or with really gentle braking and then go around and feel the center of the rim, not the disc. If you have a sticky caliper and put your fingers directly on the disc, you will burn them but enough heat is transferred into the center of the wheel where it attaches to the hub for you to feel it. If all hubs are cool, then it's probably safe to check the discs and if one is warmer than the rest the caliper is most likely sticking, a couple of hours work to fix.
These are issues I have had to deal with on the Forester so I hope they are of some use to you or anyone who is looking at this thread.
