Right, so I had my first guitar for christmas, a yamaha F310. Not the most expensive thing in the world, but it's brilliant. I first noticed a problem with the A (the 2nd highest note string, I think I got the right one) string about a month into my learning. It makes a sort of loud twang sound when I'm playing. If ever I hit it accidentally during a power chord, it makes a horrible sound, and it doesn't sound completely right when I use it during a chord (Most noticably with the G chord where it is an open string).

I thought it was the original strings, so I bought some d'Darrio (Is that the right name?) Phosphor Bronze strings which I heard from here were very good. It fixed the problem for about a month, but now the problem is back. Anyone have any idea what causes this?
the A string should not be an open string in the G chord.

I think maybe you are talking about the B string? Second string from the bottom?
Umm...the one next to the high e.


Right, it's B. Sorry, my mistake.
yeah bro, check your saddle (the white thing on the bridge) there should be a special adjustment there just for the B string you might notice it has an extra little notch taken out for that part. if it's smooth all the way across there's your problem. the b string usually does give a whole lot of problems on cheaper instruments (like mine [.
accidentally hitting it WOULD give you problems if it's not in the same key as the rest of the power chord that your playing so expect that it's not the strings problem. my b tends to be twangy or almost bell like. try putting your 3rd finger on the b and the 4th on the e in that g chord it might help. it looks like 3-2-0-0-3-3 E-A-D-g-b-e