Recently, I noticed that when I play the A note on the low E string on my acoustic guitar, the sound that I get is like a dull thud sound. It's hard to describe, but it definitely has a different quality than the other notes (e.g. very muffled, the sound trails off quicker, vibration on the guitar body is stronger). The problem always occurs on the A note, even when I tune the E string down. Is this just a problem with old strings or could something be wrong with my guitar?
It's always worth going for a new set of strings when there's a problem with guitar sounds. It eliminates one sector straight away. If the problem is still there, it could be that the fretwire at 6 is slightly too high compared with 5, thus not allowing the string to vibrate fully. Clues as to what sort of guitar strings may well help answers.