Help with Kilpsch KLF30 rattling cabinet

Anyone have any tips for repairing a KLF-30's rattling cabinet? The significant rattle is about 5 inches from the back panel's top on the right-hand seam. I can place a finger (with pressure) on the exact spot on the outside of the cabinet (versus the back panel) and it stops completely. Without the pressure the cabinet's rattle is absolutely HUGE!

Thanks for any tips! -Bob