Originally Posted by hammerdwn
Seems like you have what's called "ground loop". Your home's electrical plugs might have a ground wire issue. How old is your home? Does it use 2 wire electrical or 3 wire (with ground). Some solutions are to plug all your devices to one common power strip so the grounds all match. If that doesn't help then try other combinations of plugs until the hum goes away.
It could also be coming from a coax line that isn't grounded to your house wiring properly. You might need to have your cable/satellite company come out and ground it. Experiment by disconnecting coax lines from your AV equipment one at a time until the hum goes away.
There's lots of info about this topic:Search Results for Ground Loop
I had already gone through a lot of topics and they seem to give the same suggestion as you did. I'm just wondering why the issue is just with the HTPC because when it is turned off or when the HDMI from the HTPC is removed or I switch to other sources on the AVR, the hum is gone. I have a Corsair CX500V2 power supply and an APC branded UPS both of which are supposed to give "clean" power to the PC. Also, I didn't expect a digital interface such as an HDMI to induce noise.
A few points to note is that I had a similar issue with my 2.1 Sony speakers with analog connections and I got rid of most of the hum by getting a better branded aux cable but it's still there. However, I don't hear this on any other audio device such as the TV or my AVR with all other sources except for the HTPC.