Maybe a couple of things.
1. Did you make sure the spring end is upagainst the perch of the strut bottom, and the top of the spring is seated good against the top plate.
2. A worn swaybar link bushing, or swaybar link, and swaybar link bolt (bolt maybe loose)
3. Bushing (body to strut) the strut shaft is shifting inside the hole (a worn mount not common in the rear)
4. Plastic cover missing on bump stop to subframe (metal to metal)