Sounds like a leaking air strut to me. Mine had same symptoms / settled down over night, sometimes would stay OK for a few days, then sag. Had the same rattle in the suspemnsion when going over small depressions in road ( manhole lids, pavement cuts ). Go to Arnott Industries for rebuilt struts - $ 299 vs. $900 @ stealer. Might want to get on order as they sometimes have a backlog. Just to give you something else to think about, shortly after I replaced the left side, the right side let go.