Hello,welcome. My name isXXXXX will do my very best to assist you with yourquestion
The issue that you are describing is either the strut is now worn out and has little resistance or the strut mounts are worn out,either one will cause the issue that you are having.Raise the vehicle with the weight off of the strut and check for leaks and excessive bounce from the strut or movement from the mounts.Most warranty do not cover struts and mounts as they are considered a normal wear and tear maintenance part.
The Kia Soul only has 16000+miles on it, so wore out parts would be a remote possibility. I was under the car Friday, and couldn't see, or find anything.
Since I wrote you, car has been inconsistent with noise. Sometimes I hear the ker-plunk, sometimes I don't.
I'm now waiting on to see what the dealer wants to do, since the vehicle is still under warranty. Thanks.