Was mentioned somewhere earlier in this thread. Depends on what type of external speaker you are using. I originally had mine hooked up to a Midland (very similar looking to the standard Motorola two-way radio external speakers used for years) and it had a very loud hum. I replaced that with a "generic" external communications speaker and the hum got noticeably lower. It's still there though. Of course the audio quality now suffers, as the Midland speaker is professional grade and projected a nice deep tone with transmissions. Waiting for my in line audio noise filter to arrive from Amazon today. Will let you know if that makes any difference.