It sounds like you are hearing distortion. What you need to do next is find some well recorded music that does not have the noise and test it with your setup. Try "I Want To Hold Your Hand" by the Beatles. If you hear distortion in there then your setup probably is introducing distortion. The next step will be reducing the input volume into your FiiO. Do this by turning down your volume on your PC. If you are at 100% volume on your PC when you hear the noise turn it down to 80% and try again. Be sure you do not have any Equalizer settings as many digital, if not all, EQ adjustments often introduce distortion.
The DAC inside your PC could also have high Total Harmonic Distortion which is then amplified by your E9. Amplifying distortion is going to just make it louder. This is perhaps why the E7 DAC cured the issue for other folks. Do you have an ipod or similar device? Try the same tracks on it with your headphones and if you do not hear it then you know it is probably the DAC in your PC, if you do then it is most likely your music, headphones, or cable.
Another test track would be "Luno" by Bloc Part. You should not hear the distortion while listening to it. If you do then it is something else be it the DAC in your PC, your headphone amp, or your headphones. Unless you have damaged your headphones they probably are not the cause. Keep in mind that the DAC inside most ipods is actually pretty good and when properly amped and played through great sounding speakers / headphones the musicality can be very immersive. I use my ipod touch as a general check on test equipment because the DAC is quite nice.
http://www.youtube.com/watch?v=NYxh_x20Vrk
If your music is recorded poorly or ripped from CD improperly you can introduce distortion as well. Are your MP3's recorded at 128kbps at 16bit? If they are any less you are introducing distortion and the tracks are going to add to the problem.
Smart money is betting on the DAC inside your PC.
