Use an ohmmeter to check for unexpected connections to building ground. There should be no connection to building ground in the Balun wire. It’s interesting to measure the voltage difference between building ground and neutral. Don’t expect zero, but it should be low.

If the wire between Baluns runs parallel to power wiring, it is possible that coupling will induce hum. You may be able to measure this with a high impedance millivolt meter. Remove the Balun from one end of the connection and measure the voltage on the raw wire. Be sure to measure the resistance from these conductors to building ground too.