When transmitting on 20 meters my rig is causing interference to our home
intercom (in the wall hard-wired unit) and TV (cable). I've checked the
ground, the rig is new (FT-897), as is the antenna (G5RV-Jr). Antenna is
located in attic due to neighborhood restrictions.

I'm guessing the the intercom wires in the walls are acting as a big
antenna, but where would I start putting chokes on? Or would a filter at
transmitter (low/high pass?) be where to start?