Try simplifying the set up.
Get the UPS out of there (temporarily)
Plug both the scanner and the PC into the same outlet strip.
Disconnect EVERYTHING else that can be. (External antenna included)
See if you have hum?
If so it may be the scanner power supply. Try running it on a battery temporarily.
If not, then add connections back one at a time until the hum returns.
Then try disconnecting the connections you have one by one until it is gone.
You will then have identified it as either a true ground loop, and know what connections are involved in the loop, or as power supply hum.
Note, it can be both, and you can have more then one loop.
ave fun.
Try this with a recording studio, and you can kill a good week hunting it down.
Also, as you try thing METHODICALLY, try reversing the pug on any equipment that can be reversed.
You can also use a 2 prong adapter to (temporarily) unground three prong equipment.