My computer is too loud?

My computer is abnormally loud on occasion. More than just a working hum, or a few clicks while it thinks. On occasion it will buzz, like a metal against metal rattle. The sound seems to come from just behind the front casing. A quick smack on the side of it usually quells the sound, but on occasion will make it worse...in which case another smack knocks it down. The case (I guess that's what it would be called) is by Lenovo, if that helps in the slightest.