I have two express units. The oldest one (not old) has developed a terrible audio warble(?) Out of frustration I changed the out the older unit with the newer one and the sound came through perfect. But I didn't have all the channels come through that my first unit has on it. I put the first (older) unit on my newer TV and the sound was great. So I changed it back and the sound was great for a day or so then it started to warble(?) again. I repeated the sequence and the sound was clear again.