PoePoe2797

I have a wierd buzzing at idle. It likes to go on and off. I will be sitting at a stoplight and its kinda faint but I can feel it through the pedals. It would just start buzzing for 1 sec, stop for 1 sec, and buzz again for 1 sec.....basically random, but the buzzes are always the same. I'm not sure it it goes up in the rpms because its too faint to hear when I rev. I tried looking under the hood and everytime I look, the buzz is gone. Any ideas? Oh, I just replaced the a/c belt and got a new idler pulley, maybe I might have retensioned it wrong???? What does it sound like when the belt is too loose or too tight?