I just got a Behringer X18 mixer, I got an incredible deal on an open box unit from Musicians Friend. This thing has all kinds of potential, but it's possessed. Randomly it emits ear splitting sounds like you'd hear in a sci-fi horror flick that stands my hair on end and makes my skin crawl. I can't find a trigger to the sounds. Before I wrote this I disconnected the iPad from the access point, so there's no connected devices, and I still get the sound regularly. Currently I'm running 2 mics into the mixer, along with my bass through an M80. For mains I'm running 2 older Behringer 15" passive speakers. To power those I'm using my old Behringer powered mixer, running the main outs from the X18 into my powered mixer's "Input to Main" jacks. Then I'm only using the main master volume knobs on the powered mixer and have turned everything else down. Basically it's just a large 1200 watt (that's Behringer watts, not real watts) power amp. I also have 2 powered speakers on Bus 1 and Bus 2. I can get the sound through all the speakers. We rehearsed with it yesterday and for 3 hours didn't get any sounds, but today I'm getting it regularly. I've attached a link to the sound file, turn down your speakers before hitting it. Anybody else ever experience this? I don't know if I have a bad unit or some sort of random electric noise source.