Amp making speaker buzz?

-I hooked up an alpine 6x9 to an amp a friend gave me. He knows nothing about audio for the record. The amp says 180 peak so I figured around 60 rms. My 6x9 is 85 and when I hooked it up I could hear a little music and a loud obnoxious buzzing sound. when I turned the gain up it got louder. So I unhooked it and hooked the speakers back up to the head unit and the speaker is fine. What's wrong with the amp? I used the same exact wiring for a different amp and it worked fine.