My first thought would be an out-of -adjustment brake shoe, or sticking caliper.BUT... i thought this was my problem on my wife's neon after adjusting my drum brakes (which i had just replaced, therefore making them the foremost suspect). finally figured out it wasn't the brakes. it was very loud, and very intermittent as you have described. it drove me crazy trying to pinpoint it. Turned out it was the damn rear strut assemblies! i found out by bouncing the back end with my hands. the sound was exactly as i was hearing from inside while driving. it seemed to happen more when i drove the car "spiritedly", instead of when my wife drove it, but kept doing it off and on for awhile-air in the leaking struts (?). funny thing is, it stopped shortly after i figured it out. anyone else out there ?