Hello and Welcome to JustAnswer.com! If you are POSITIVE that this is not a tire issue, I would suspect a wheel bearing. This best identified by swurving back and forth while going down the highway. If the noise seems to vary, it is most likely a wheel bearing. You can then Identify the location of the wheel bearing by raising the wheels off the ground and running them. Then listening closely to each wheel, an automotive stethoscope works great for this. If you have any questions or need further assistance, please feel free to ask. If this information was helpful, please click ACCEPT! Thank You.