Hi, I have been using an aggregate output in Logic that's been working fine but have run into a problem recently when I removed one of my 3 aggregated devices and changed it for a firewire output device (Behringer FCA 202) and found there to be a loud ringing noise (perhaps like a USB ground loop ring) coming out of the Behringer's ouput. The Behringer & my other interfaces do not have this issue when used outside an aggregate setup and so I think I can narrow the problem to within Logic, but I have experimented with everything I can think of within Logic & the Audio MIDI Setup. I have previously had the same issue when I aggregated my Macbook's headphone output with other interfaces, so I simply took it off but I can't understand why it might do this with the Behringer. The other devices are a Focusrite Saffire 6 USB and a Mackie VLZ USB mixer. The Behringer replaced a Roland TB-3 because the TB-3 only works at 96kHz whereas the others can all do 44.1kHz. The problem is only happening when the aggregate output is combined of USB outputs and non-USB outputs (ie FireWire or laptop headphone) so I am wondering what it is about combining USB and non-USB outs that causes this noisy squeel. When I selected aggregate output in my System Preferences as a general Mac audio output the same noise did not occur so everything seems to suggest it is Logic that has an issue. Any advice would really be much appreciated.