suggest go TCM and get them to trace and "insulate" the rattling areas, sound may not be from the arm rest. i had a similar problem and got it fixed recently. my few weeks old car had some soft but still audible rattling sound when driving on roads that're just slightly uneven (sounded like from dashboard, door panels, passenger seat, armrest areas), very annoying. no sound when on smooth roads, e.g. some nice flat stretches along expressways.
reported the problem during my 1k servicing and was told the mechs insulated the dashboard and door panels when i collected my car after 6 hrs. rattling sound gone but only for a few days. so i sent car back to TCM and left it there for 2 days. TCM did a more thorough job this time, irritating sound gone and its been more than a month since (hope it stays that way). mech told me that the sound is caused by car’s interior parts rubbing against each other when car “shakes” while moving on uneven roads. they added insulation tapes between the rubbing parts and rid the rattling problem (wonder how long the insulation will last). but of course, the normal “squeaking” sound when car "boogy woogy" over more undulating grounds or over humps are still there.
good luck
