yeah, you ever heard the wheel bearing going out in a car or on a trailer?
sounds like "skrk....skrk...skrk...skrk..skrk..skrk.skrk.skrk.skrkskrkskrkskrk" as you speed up. lol best imitation of the sound i can think of. eventually, when you are going fast enough, you cant hear it. then your wheel falls off, lol.
the sound might be from the deterioration of the sleeve bearing as nagasama said, antec uses sleeve bearing fans because its cheaper than ball bearing, but if running from a horizontal position the fan loses its lifespan and signs of it become apparent
Id be pretty annoyed if the top fan fails, ive only had this case a matter of months, sure its on everyday but that would be taking the p*ss if it does fail, especially as you cant buy 200mm fans.