I would suspect the source would be a combination of SMPS and shielding.
I find Antec computer cases to be pretty fair at shielding. Enlight was also but
I do not like the newer cases. The most likely source of RF of any power would
be the power supply. Other sources tend to be much lower power and ranged.
If the power supply is using SMPS, it is a significant RF generator. And naturally,
it would impact Amplitude Modulated (AM) signals far more than Frequency Modulated
(FM) signals. Try placing the computer in a marked, central position. Take a portable
AM radio and walk around the computer. Note where the signal is weakest and where strongest.
assuming you are not running the cpu with the case half dis-assembled, you will find that this experiment
will probably indicate the rear of the case. If you had a spectrum analyizer handy, you can tell the
strength of the signal and the spread spectrum frequencies. But I would suspect the source is
probably the open (grid) area around the exhaust fan of the Power Supply. If so, take a small
disposable pie pan (Metal, aluminum will work) Turn this into a little duct that redirects the
fan output upward but otherwise encloses the opening. Use metal tape to attach, as it does nothing
unless it acts as an extension to the conductivity of the computer case.
If you are an experimenter and have many knock-outs missing at the rear, or the front assessory
area then YOU are the source. Your cpu box is a Faraday cage for a reason.
SMPS - Switching mode power supplies in depth coverage :