For the category "better late than never"...
Before I left for Texas for a month, I disconnected the antenna cables to my Ten Tec Jupiter HF rig and my Yaesu FT1900R 2 meter rig. When I got back and reconnected everything, I had major QRN on 2 meters. People could hear me fine, but reception was spotty at best.
I tried moving the cable around with no success. The only difference from before the trip is that central air conditioning unit was on. I never got motivated to get into into in detail, because HF was fine (I thought) and I use 2 meters more for mobile.
This past weekend we had an ARES drill for which I was assigned to operate my base station. I turned off the A/C--no difference on 2M. The next suspect was the computer, but that's always been right where it is. I shut the computer down and the static disappeared. I looked online and found that the monitor may be the problem. I tried just turning off the monitor without shutting down--that was it. Brain jock that I am, I ran the antenna cable behind the monitor. I moved it, running it under my desk to the Yaesu, and that was it.
We were also operating on 40 meters. I was getting static there in about a 3kHz range. Since the radio gods have a sense of humor, the frequency we used was right in the middle of that range. It turns out that the source was the A/C--when the unit switched on, I got static.
Moral of the story: don't run antenna cables behind a computer monitor, and A/C has potential to produce interference.