There are certain frequencies in every radio receiver that will always act like an "open mic" situation is in effect because that frequency happens to be mathematically in the wrong spot in the radio, due to the way the radio receives signals. I don't have the info in front of me to detail it in this post, but if you Google "birdies" or "intermediate frequency" (both with reference to radio receivers), you'll get more info.
The Intermediate Frequency Exchange makes the internals of the radio shift the intermediate frequency slightly for that particular channel/frequency, which should allow you to receive any signal that is on the channel.