Could have been a very very small piece of excess rubber on the tire leftover from the manufacturing process. Might have just worn down after driving for a while?

 

Wheel weights hitting the caliper. I had this issue with a set of OEM 18" wheels on my big brake kit. The mounting guy had to use 2 layers of weights and they were slapping the caliper as the wheel rotated. It stopped likely because you wore it down enough that it is no longer making contact.

Give it a look next time you have the wheel off.

 

could be the meat curtains.

 

Did you upside your tires and they caused rubbing on the liner and it went away when it wore enough