Mystery thumping noise

What in the heck is that thumping nosie?

Sometime before lunch I began to notice a thumping noise coming from the driver's side front wheel area. It was very irritating because that nosie cannot be a good sign. We stopped and looked at the tire before leaving the rest stop but, we didn't see anything. The wheel seemed to roll funny.

We stopped again but still couldn't figure out the odd noise. I have a very bad feeling about this, however we press on to Arizona.