I live a mile out from Midway Airport Chicago and have been monitoring Midway tower for over 30 years. A little over a month ago the tower frequency (118.7) has been broadcasting a carrier signal similar to having an open key on the channel. When either the tower or an aircraft transmits there is a whistle (again similar to an open key) during transmissions and has been very difficult to monitor and sometimes you can't even hear the aircraft transmissions. This cannot be squelched on any scanner that I use so you can't scan other frequencies becasue it locks on the channel. Does anybody have an explanation as to what is or has occurred on the Midway tower frequency?