suspension can't seem to hold the weight of 3 people in the back

My 1.1 automatic daihatsu charade makes a grating sound when it takes a lot of weight in back seat like 3 passengers. When I drive the car with 4 adults 2 in front and 2 in back its fine, no noise, the moment I put an extra adult in the back it makes a loud continuous grating sound, please help as I have taken it to many people some say the body needs to be higher, some say the shocks i do not know what to do, I did however have a small accident I knocked the wall and the back of the vehicle got smashed a little, but it was repaired, the boot lid bumper and the vehicle was stretched properly underneath.
Its runs nicely its just this noise.