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There are quite a few things that can cause noise over bumps.
The inner and outer tie rod ends as well as the steering rack itself could cause the noise.
The sway bar mount bushings or sway bar links could be making the noise. The links go between the sway bar and the struts and have a ball joint on each end that can loosen.
The lower ball joint in the lower control arm or lower control arm mount bushings could be the cause.
The strut itself or it's mount can make noise over bumps.
Loose fasteners on any of the components will make noise also.
I'm assuming that they've at least inspected all of this at some point but if you're still having noise then it will all need to be inspected closely. If they can't find the noise though driving it and inspecting the components then they can use a tool called Chassis Ears to help find the source. Chassis Ears uses a handful of clamp on microphones that can be placed wherever necessary and they transmit to a receiver with a speaker/headphones and the user can switch between channels to see where the noise is the strongest.