One of the speakers in a 2x10 cabinet started buzzing, regardless of volume. It's not super loud, but definitely noticeable when I'm practicing. If I lightly touch the speaker cone (bad idea?) it stops immediately, but starts again if I take my finger off of it. It sounds almost like a cable or something is touching the cone, but I removed the speaker from the box and don't see anything obvious. Thoughts on what to try next? How do I troubleshoot this?