Hi Nick I've seen this happening on a MDD G4 system with Mac OS X 10.3.x too. I've not yet got to the bottom of it either - it seems to happen entirely of its own accord, and the only way of getting rid of it is to relaunch the audio engine somehow. The system I saw it on was a G4 1.25GHz Dual, 1.75GB RAM, Mac OS X 10.3.2 (I think), Logic 6.x.x and MotU PCI424/2408mkIII. Good luck, -- The best of times to you all... Pants