I would suspect things like exhaust hangers or loose shock bolts.
I would jack the car up and put jackstands under there and then start grabbing anything I can see and wiggle it to see if it is loose. If it is find out why and tighten it.
I do this routinely and you would be surprised how often you find soimething loose.
Also of course check to make sure nothing is rolling around in your trunk, check under the floor board in the spare tire well too.
Another possiblity is a loose or sticky caliper.
Good Luck,
Jim Henderson