OK, if anyone’s still here, the problem seems to be due to the noise cancelling mic picking up sound from the earpiece/speaker. It’s a common problem with non-stock ROMs and, as far as I am concerned, the main barrier to sticking with them. Recently I had to go back to stock on a GT-I9300 because I intend using it as a doorbell/intercom. This necessitates use of the speakerphone setting which produces intolerable feedback on /e/. Going back to stock fixed it immediately. Being so outdated brings its own problems but that’s a story for another day.