caliper rubbing on rim
 

2005 mercury mountaineer 4.0L AWD.

My caliper or the housing started grinding on the rim. it has made a nice, polished circle completely around the driver side rim only. any suggestions on were look first. I noticed when I changed oil this weekend and took tire off physically grabbed all the components and everything felt tight. there is something loose since it does it every once in a while. has this happened to anyone else? if so, what was the solution. I'll probably block it up again this weekend and look deeper.

thank you in advance.