Tony - not sure if this is common to the 120's but in the 150's a common issue as you described is the little plastic guide (black V shaped plastic) for the rear door wears and creates an annoying rattling clunking type of noise. Maybe worth checking out as it's a low cost item and if this does the trick for you - you'll be sorted. To see - just wedge some paper in and go for a drive and see if this is the one?
It will be this, I had the same issue, replaced it (cheap, below $30.00) noise has gone.
Also, check your tow hitch, this to can make a noise.
And I know you checked your door, but the hinges let go to easy, try only to just open the door ajar and hold it at the bottom and try moving it up and down, if there is play, that's your noise. Do this in an arc until fully open, just to be sure.
Cheers