Some other possibilities might be to make for sure that both the computer and scanner are on the same ground (or any thing else hooked to the scanner or computer). For example they are all plugged into the same outlet. Although once when I lived in an older house I actually had to plug the computer into a different outlet than my scanner. Not really for sure what was going on there, maybe my surge protector was the problem.
Also you might listen to the scanner without the computer attached to see if the hum is still there and do the same with your computer sound card to make for sure that one does not have a problem already. I found I had to reset my sound card since I was getting a low level hum from it without my scanner attached and in another case I found my antenna was the problem.
However, once after 3 days of trying everything I could think of, I found that the PD was having technical problems and they were actually broadcasting the hum (one of the units asked why there was hum on the signal). I sure wasted a lot of time on that one, so having people let you know what they are hearing will be helpful.
Good luck hums can be tricky sometimes to eliminate.