Mine has been in the SC for this since last Friday (8 business days) and at this point they have the HV battery out! The rattle on mine originated when they successfully fixed the Vampire Drain, and it is suspected that the work they did may have dislodged a hydraulic line clip - which makes sense with respect to the type of rattling that can be heard. Apparently there is a 1.5" space between the vehicle floor and the HV battery, and there are a bunch of hydraulic lines routed in that space.
They suggested that it might be fixed by tomorrow, I will post a follow up when done and include the final diagnosis if they will tell me!