When you get a chance, have a close look at the clips on the springs, the ones that hold the leaves from splaying sideways. One may have come loose of fell off.
U bolts tight?
Tapered shims broken?
Try spraying the entire area, frame, hangers, springs, shackles etc with a thin coating of flat black paint. Drive it a little, make it do it (DW) - then park it on wet grass overnight or where it'll be humid.
Next morning look underneath for a thin orange line of rust - where something's moving that shouldn't be. Called a poor man's magnaflux.
The movement breaks off tiny steel particles - dust - and pushes them out. The humidity makes that dust rust. The black paint helps you see it as an orange line against the black..