2002 Ford Taurus SES

I'm leasing a 2002 Ford Taurus SES. Whenever I haveat least one passenger in the back seat it sound like the car bottoms out. I had it to Ford service (6)
six times for this problem and it never gets fixed. They replaced struts, bushings and added rubber baby bumpers to the muffler pipe. I still have the sound of ca-thunk, ca-thunk when I go over small bumps. The car has 6 thousand miles on it and it rides like crap. I've had 2 other new Ford Taurus's and never had a problem with either of them. Has anyone heard of this problem before.
Thank you in advance
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