You'll have to isolate the noise. You are saying there is noise, even when there is no source? Is there noise when the input cables are disconnected? How much noise? Most systems will have a little bit of noise, even self-amplified studio monitors. The thing that matters is will that noise be audible at sound levels you are going to listen to. If you turn those monitors all the way up, you will probably hear something, even if nothing is connected to them. A little noise is acceptable in heavily amplified signals.
What sort of noise do you hear? Is it a soft staticy noise or is it a low frequency hum? Is it a hiss? The M-audio box should be fairly quiet. This leads me to think that either you don't understand what kind of noise levels are acceptable, or interference is getting into signal but it is occuring past the point of the source.
How long is the cable which connects the M-audio box to the monitors? Is it running alongside and power cables? If the cable is running parallel to power cords, it can easily pick up electrical noise like that. If the cable is long, it can act like an antenna and receive interference.
If there is a hum even when the signal cable isn't plugged in, you might have a grounding issue with the power cable or your electrical outlet.
Remember that some noise is expected when you turn the volume all the way up. You just don't want to hear any at all at the sound levels which you are ever going to listen to.
