I have had the same issue. There are a couple threads regarding this, one where a member took his car in and they replaced the trans axle. If you can get your car to do it consistently, take it to the dealer ASAP. I am not saying its an emergency, just because the sooner the better. I took mine in, but could not get the noise to produce, so I am stuck with it.
Some people may think its the brakes and will swear that its normal (and it may), but I have 6k miles on my car and I still get it every once and a while. Mainly at 2500-3500 rpms, no brake applied and can feel it behind my clutch pedal. But, lately it only occurs when I am "playing around" with the car.