RFI from steel TV bracket?

My first thought is; it's not the rack but the TV. Try lifting the ground to the TV. Or possibly pluging the TV into a known different circuit that is on the opposite phase of the balance of your system. One other suggestion if you feel it is the rack, can you put the old TV(if you still have it) on the steel rack and determine if it is indeed the rack. And fiannly I assume you have tried unplugging the TV to see if the hum diminished?
I have been down this road and you just have to try every possibility to find the gremlin. Good luck.

If I understand this you just installed the TV mount onthe wall. Is there any chance a screw cut into your Romex in the wall and is hitting a grownd wire? I would say try removing a screw at a time and check for the hum. It just odd that an LED Tv is doing this or that the metal mount is causing it.

What all do you have connected to the TV? Are you using a Pre-pro that you can connect everything to then just run HDMI from the Pre-Pro to the TV? If you are running analog IC's to the TV and they are picking up interference from the AC cable then maybe a shielded power cable would help. If possible try to have your analog IC's cross the AC cord at a 90 degree intersection, if they must cross. If they are coming out of the TV mount area and must be together, try sheilding them as much as you can.