Chrysler Town and Country noise

Over the summer I bought a brand new Chrysler Town and Country mini van. It has 11,000 miles on it and for the past few weeks it been making this loud noise. So loud that you cant hear the other person sitting next to you talk unless they speak loudly. I took the van to our mechanic and he said that it was the back tires making the noise because there has been to much weight put on the back tires. No one sits in the back of our van cause i use it to store my groceries from when i go shopping and stuff so i cant believe that my groceries would weight that much that my tires are making the loud noise. My question to you is if you have heard of other people who are in the same situation as i am and if it really is my back tires or is it something else?

What does the sound remind you of, btw this should go to the dealer for repair under warranty, If your dealer sandbags you go to a different dealer for problem resolution. It sounds like a serious issue to me, and I doubt the tire issue.