Donohue96M6Z28

ok this is a little confusing to explain..but in the mornings when i first start driving my car...it seems like theres these weird sounds when i go over bumps...it seems like its coming from the plastic on the sides of the back seats..it kinda sounds like icechests...it sounds like ice just movin around i dont know its really weird...and its only at first...after about 5-10 of driving..when i go over the bumps the sounds gone...i know its kinda hard to understand what im trying to say ..but...any ideas?

shoebox

Possibly coming from the shocks, before they get warmed up or maybe their rubber mounts?