+1 on grinders suggestion to physically ground the two units together.
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This may or may not completely eliminate the problem.
If it doesnt , goto a car audio store and buy a 'ground loop isolator' , they are about 25$......
I had to do both myself , 100% hum free now.
edit @ EE :
The problem is 'ground loops'.....While the concept of 'ground' suggests they are all at the same potential , they rarely are.....with sound equipment , the 'chassis ground' and the 'signal ground' are often different.....the hum is the result of these different potentials.
More info :
Ground loop problems and how to get rid of them