On the door: Is the trim coming off? If it isn't, then you can always pry is off and check inside the actual door. Also, if you're sure the actual door is on tight, you may wanna check the rubber insulators (both the insulation and the dampners) You may have lost one and its clingin now.
Also, have the back tire checked, or try to see if you lost a weight. Throwing a weight can make your tire wobble crazy-like, and although i don't think that would be the problem, it could make something loose on the underside of the car clangle about.
Not necessarily on point, but I had a rattle front the front right side that took me 2 years to get rid of. After changing struts and sway bar links and playing with the brakes, I found the folks in Chicago had given me a spare door hinge pin in my fender well.