Oscillating sound in front of car
 

I have a '05 3s hatchback and enjoy driving it every day. It has 36K and I replaced the tires at 25K with the BFG G-Force Sport which I rotate on a regular basis (as recently as two weeks ago).

I've noticed that when driving at speeds of 50mph or higher that I hear an oscillating sound from the front of the car, like a tire is out of balance. However, no shaking in the steering wheel or vibration in the seat so that leads me to believe the tires are ok. The second thing I noticed is that the sound diminishes (slows down) when I press the brakes and picks back up when I release the pedal.

So, I speculate it may be the brakes or rotors. Any thoughts? Could it be something else maintenance related (like a bent rim) or alignment or something more serious (a bad part)?

I ask all this because I have a trusted mechanic that I use for maintenance and will deal with the dealership for warranty work and I'm trying to determine who to talk to first.

Thanks in advance!
Chris