Assuming the hum is heard by the remote people, it's possible there is a hum picked up acoustically or electrically into the system and transmitted to the far site. When the remote people are talking it is likely that the AEC is providing some suppression so you don't hear the audio from the local site but once the remote people stop talking and the local people start talking (or the noise floor is high) the local site audio will be sent back to the remote participants - hence hearing the hum again.
Also check that the inputs/outputs are properly terminated as balanced audio signals to ensure there's not a floating ground or something else going on.
