Pc is making buzzing noise.?

It started as a short burst of buzzing, it sounded like a chainsaw or a dirt bike revving up then it would stop. then it would do it again 1 or 2 minutes later. now it just starts up and keeps going for about a minute, stops, and 1 or 2 minutes later it starts again. i checked the fan and no wires are touching them and they spin just fine. its a rebuilt pc probably more than 3 years old.