Try to do exactly what you do, but with the car in park (that is, rev the engine about as hard as you would to make the sound, if it only happens when the engine is warm or cold, make sure it’s under the same condiditons). If it still makes the noise, it’s not the brakes. Is there a steep hill around? If there is get up to the top (but with the the car heading back down). Get the car to a complete stop, put it in neutral, shut off the engine and take your foot off the brake. IF it get’s going quick enough and makes the sound, this would help isolate the problem to the brakes/suspension/CV (in other words, it would help rule out tranny or engine problems). To me this doesn’t seem like a brake issue, but it’s really hard to tell with out being there. Also, if you take the wheel off and putz around with the break your self, remember the discs will be HOT if you’ve been driving around. One more thing. Try to get a friend to stand outside the car (maybe even kneeling down next to that tire) when it makes the sound. There is a chance it just SOUNDS like that’s where it’s coming from.