so recently my D string (4th string) has started buzzing and sounding weird whenever I hold it down on the 7th fret. I just wanted to know if anyone knew how to fix this.

Ibanez RG350EX
Yamaha FG730S
Peavey Vypyr 15 watt
Korg CA-40 Chromatic Tuner
Fender 351 Medium Guitar Picks
I have a RG350EX to. I had the same problem awhile back and found that my action was to low. I had the fret board inspected and it was perfect, so at least in my case it was the action.
I'd first suggest checking/adjusting the action, too. If it's too low, the string may be touching another fret besides the 7th when you strum it and giving the buzz.

If it's not that I'd think it must be the fret being weird or something.
Arms Of Empire
^ ^ ^ Band I am involved in ^ ^ ^
Please check it out? ;D

Axe FX II + EBMM JP6 + Jackson DK2S