If you haven't done it already, try using a different audio cable between the scanner and the computer. A bad audio cable can cause hum, and a cable swap can be an easy fix.
If that doesn't fix it, try adding an isolation transformer to the audio cable running from the scanner to the computer and see if that gets rid of the hum.
If the isolation transformer doesn't get rid of the hum, try adding a three-prong to two-prong "cheater" to the computer power cord
on a temporary basis. If that fixes it, then you've found your problem.
Are you powering your scanner from your computer or a separate USB power supply?
Sometimes, it takes a combination of solutions to totally get rid of the hum.
Alternatively, you can use RadioFeed for your feed software; it has a notch filter function you can set for the offending hum frequency/frequencies to knock the hum out of the feed. The notch filters work very well when they are set to the proper frequency. However, you'll want to try eliminating the hum by the other means first.
Let us know what you find!
