Is the noise a constant hum? If it is such a noise, I suspect a faulty or poor performing power supply. Check the power supply that powers Edirol sound card for any improper ground. Another custom made power supply which is properly regulated (voltage) and at a higher ampere rate would be an idea to consider. I think your main issue is grounding because you say the case become worse when the MacBook PSU is not there. Because, you are loosing a ground path when you remove the PSU. Then it solely depends on the Edirol ground path. If that path not a good source of ground, there lies your problem.
Is the noise a constant hum? If it is such a noise, I suspect a faulty or poor performing power supply. Check the power supply that powers Edirol sound card for any improper ground. Another custom made power supply which is properly regulated (voltage) and at a higher ampere rate would be an idea to consider. I think your main issue is grounding because you say the case become worse when the MacBook PSU is not there. Because, you are loosing a ground path when you remove the PSU. Then it solely depends on the Edirol ground path. If that path not a good source of ground, there lies your problem.
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