speaker chatter

When driving my 2000 GC, I pick up a chatter noise through the radio speakers. If I shut the radio off, then the noise goes away. The issue seems to come and go. When my battery died about 11 months ago, the issue was really bad. After the battery was replaced, the issue went away for about 10 months. Now it's back. The funny thing is that it doesn't always happen. Some days = no chatter; Other days = chatter. I've checked the charging system, the batter and the primary electrical system connections and have not found a problem. Does anyone have an idea of where to search next? I'd appreciate any help or input possible. thanks:banghead: