Where are you getting power for the scanner? I forgot to tell that that cable should have a bead on it, too. Inside the computer is a pretty harsh environment and I think there is at least a 75% chance that is where it is coming from. I assume you do NOT use this computer for anything else? I would also NOT power the radio from the computer switching supply, at least not without a robust RFI filter, like a Corcom ( http://www.zasales.biz/3EQ1.html
). They are designed for line voltage but work fine for DC, and I always use them on the output of any switching supply that is powering radios. You only need to use one leg to filter the V+, V- and cable shield being to the case and ground, so one COrcom can filter two power supplies as shown in the photo below, where we have twenty radios and six busy servers in one cabinet... it doesn't get more harsh than that.
In any event the first test is to power the radio directly, unplug the audio out so you can hear the speaker, turn off the host computer and see if you still hear the interference. Obviously if it is a workstation you would hear sound related to your activity, or if you turn the monitor on and off, etc. I have a monitor with a power supply that throws stuff out around 129.4 when it is on, and that's radiating out to the antenna 30' above. With your attic installation you could be getting crap from anywhere such as other computers in the house, your WIFI... that reminds me... if you have a WIFI access point shut that down. I had one on my tower that was throwing in random crap, ramped up when a device was connected, and it had been so many years I had forgotten I had the damn thing up there. It drove me nuts for weeks until its power supply failed (which I didn't notice at the time) and the noise mysteriously went away. I assumed it was something in the neighborhood and that was that.
When I finally noticed the AP was out (I have two, one in the basement and one on the tower, so I didn't notice it at first) and diagnosed to the power supply, I happened to be listening to my feed when the power was restored to the AP and presto... the noise was back. I relocated the outdoor AP and the problem was resolved. So the lesson is, it can take time, perseverance and sometimes a bit of luck to track these things down.