My Guitar Buzzing

Man after SOMEONE modified my guitar, my guitar is now buzzing when it is being strum really hard. The reason why SOMEONE modified my guitar is because my sister wanted a folk guitar as she wants to learn guitar but SOMEONE don't want to buy a folk guitar. So I can say that my guitar is now being a testing item for SOMEONE. SOMEONE even bought a strap for me even though the strap that I have is better (quick release). I actually hate it when the strap is tied to the neck of the guitar as it will always slip. Anyway I don't really use that guitar anymore so what ever testing that any 1 wants to do can carry on.