Strange sound

I've just tried to find the source of the strange sound coming from the pass. rear fender well/gas fill area. I can only hear it when I'm slowing to a stop.
Checked all the usual stuff....light housing, filler cap/line, bottle jack area and platform, and everything else I could check underneath. It sounds like something has fallen down in the body below the filler cap area and it rattles around. I can only hear it while going slow cuz of tire noise. This sound familiar to anyone?

pebble in hub cap? creaky flexing wheel trim?
try to find a wall to drive next to with a buddy out there.

//bc