rcpileit-RCU, i have the exact same problem you do! iv opened the unit up several times to try and find the problem, maybe about 5-12 times. its got something to do with the fan becoming old, because it starts to vibrate more violently. To understand what I'm saying, open yours up, and take the 2 small bolts out that hold your fan to the unit.{with the unit turned on} once you do this the noise stops, and is quite as butterfly. once mounted back on, the violent humming starts again. So i was stuck, trying to figure out what to do. after a long time of thinking i came up with this.....if you leave your unit running for about 30-45 seconds the humming stops, as if it was just trying to warm up, which some other people say it has to do. well it never did this when i first got it, which tells me theres got to be a little peace of dust stuck somewhere inside the back plate of the fan. for right now my accu cycle plus is for sale, because i just bought the Elite, awesome unit!! maybe one day ill take apart the fan and figure out whats going on.