proteus 2000 spdif

I am havibg a problem with the spdif out of my proteus 2000.I am sending it into a motu 828 mark 2 which is connected by firewire to cubase 4.5.1 in a mac dual processor g5. In the background to each preset that I select is a rythmic scratching sound like a pulse that holds for the length of time I hold down a note. I think it must be some kind of interference or digital incompatibility.The motu is operating at 44.1mhz . Does anyone have any ideas how to stop this.I am only usig the digital out because the analogue outs have such a low output volume. Alternatively is there any way of increasing the volume internally?

thanks
Monty

Try setting the motu to "external clock" oder "slave mode".