Rear deck ticking noise due to body flex

[toyotavios_11]

Owing Yaris sedan 07 model. Recently notice that there is ticking noise from rear deck whenever the car is over rough pavement. Initially thought that it was due to rattling of interior panels, so removed all the rear panels including seats and seatbelt mechanisms to diagnose the source of noise, but ticking is still there. Hide to the trunk to check the noise when someone is driving for me and notice that the rear seat striker panels which welded to the rear deck are making the noise due to body flex.

I try to "crack" the five spot weld points of each striker panel but useless and now im clueless to solve this.