Just to make sure its not a rod or valve related, open the hood up and have someone rev the engine a little, if you hear the rattle that changes its frequency with RPM, then its engine related, if you hear nothing then it could be a rattle inside your cabin.
To me it sounded as if it is engine related and you did say that you only heae it when you press the accelerator, and just because you hear it at 120km/h and above doeasn't mean its bot there.
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