MY 124 chassis wagon has a squeek and occasional creek/groan from that same area. For the longest time I thought it to be the rear suspension - probably dried out old bushings. It turned out not to be the suspension at all, but rather the folding rear seat. All the noises disappear when the seat is folded down. I've not yet dug into it to see what the problem is with the seat, but try folding yours down and see if the noise disappears.
My personal experience is that suspension noises generally go away in the rain, rather than surface. The water temporarily lubricates the bushings, quieting them down.