flaky talkback?
 

When I first got the control 24, I was using the internal mic, which sounded like crap. But then I plugged in an external (a shure SM-55, very retro looking) and for a few weeks it sounded very clean. Much better than the internal. Now, out of no where, there is a bad buzz in the talk back. I never turn the unit off and the cable used to plug it in is in fine condition. Any idea what could cause this out of nowhere?