would that cause the main amp to buzz? or just the extension speaker. i've ran stuff through the extension alone and it's fine.

I recently found a Crate IIR with extension speaker for 60 bucks at a music store in boston. It sounded good at the store, everything worked fine. But sometimes when i turn it on now, it has this buzz. The volume on it is turned down all the way, and it still buzzes. Same if nothing is plugged in. I'm not sure what's causing it. Sometimes it's louder than others. the extension speaker doesn't buzz. Not sure if it's a bad connection, or if the electronics are too old, or if something is touching something else. any thoughts?

thanks