I played at a bluegrass festival last night and had an interesting experience. Every time I played a B on the 4th fret of the G string I got this rattling echoing feedback. It didn't happen on any other note but was very noticible even if I barely plucked the string. Eventually we moved to keys that didn't require This made me wonder about two things. What causes feedback and why would it only happen on certain notes? I await your wisdom.