Got a problem: No audio, loud pops.

I have a fairly simple setup involving a MOTU 896HD with a Firewire
connection to a Mac G4 double-processor workstation. I have mikes into the 896HD and powered studio monitors and phones on the outputs. I'm using GarageBand 2. The setup has been working perfectly until this past week. (Someone may be fiddled with the setup when I wasn't there.)

The problem now is that I can get no audio on playback (monitors or headphones), although I can still apparently record. There are pops and high-pitched distortion when playback is attempted. However, iTunes will play back through the MOTU fine, as long as GarageBand is shut down, but not if GarageBand is running. I have checked sound System Preferences, which list the MOTU as the input and output defaults. GarageBand preferences show 896HD as input and output sources. FireWire Audio Console lists clock as "Internal", optical in and outs as off, sampling rate as 44K. (I'm not near the setup now, so I can't remember the rest of the settings.) I tried making Macintosh Internal as the clock reference.

Going through the MOTU manual, it seems to indicate that this situation likely originates with a lack of clock sync between the Mac and MOTU, although another place in the docs indicates that just for playback and record, you don't need to sync.

I have re-installed GarageBand 2 and the MOTU utilities and rebooted with no change. Mac disk utilities report no hard disk problems. I can change the Mac output to the Mac on-board speakers and I get proper output from GarageBand. It certainly seems to be a syncing problem from Mac to MOTU, but no changes in either preferences settings boxes have made a difference.

I appreciate any help you can render.

B.