Front left noise?

I started to hear a noise coming out from front left side during normal driving. The noise can be heard all the time, but it is more clear on the highway. So it is speed sensitive. The noise is like whining or harmonic high frequency, low-pitch noise. Like UUUU(high)UUUU(low),,,,The penger can hear a little bit from right front as well but much lighter. The dealer checked and said it is just tire issue. But my car is only 4000 miles on it. What else can be wrong? It is also a little bit road dependent. On some highways, the noise is lighter compared to others. But it is defintiely still there. Thanks.