Car makes clunking sound when people are in backseat?

I have a 2003 mustang v6 and whenever I have people in the backseat my car makes this horrible clunking sound on the right side....usually when we hit a pothole or a bump in the road...but I had 4 people in the car last night and it was doing it when I was accelerating...i've heard it could be my shocks...but whnever I have only myself and another penger in the car it doesn't make this sound....it almost sounds like something is hanging from the bottom of my car and hitting the ground....what do you think?