Cheap imitation

My Powerbook Produces A High Pitched Tone

Hi, all. I have the following issue with my Powerbook G4 and MOTU 828 MkII audio interface (which also includes a 1x1 MIDI interface). When the MOTU is attached (via firewire) to my mac and is switched on, then each time I start Logic (Pro 6) my Powerbook begins to produce a high pitched tone (of about 12kHz, I believe, but it could as well be 13kHz or so). It does not stop before I quit Logic. This happens also with other audio/midi applications like Audiodesk 2 or Steim Lisa X and with both my 828's (I have two). With applications like iTunes or Quicktime player, the Powerbook outputs the tone as soon as an audio file or movie file is played back and ends doing that when playback stops. Furthermore, when opening the Audio MIDI Setup of OSX.3, the mac starts to produce that high pitched tone only if the Audio MIDI setup displays the MIDI devices. If it displays the Audio Devices, the mac is quiet. However, when changing the Audio MIDI Setup to display the MIDI Devices, the mac starts to produce its high pitched tone. When changing back to Audio Devices, the tone does not disappear. However, it does disappear some ten seconds after quitting the Audio MIDI Setup.
While this high pitched tone is not loud, it is clearly audible and as such very annoying.
Does anybody know why my Powerbook does this and how this can be fixed? I tried to delete the Motu 828 MkII's from Panther's MIDI-setup, but this seems not to be possible. Is this perhaps possible in Tiger?
Thanks! The cheap imitator.