AC3 Background noise

[KaBeCi]

I'm using lastest version of AC3Filter (0.6?) to decode AC3 5.1 into 4 speakers, and after watching some DivX and XviD movies with AC3 audio I realized that ALL of them have the same background noise. It doesn't happens with those with MP3 stereo, so it's not my amplifier/speakers. And it only happens when the movie is in "play" (not when is stopped or paused).
I supose its the AC3Filter, but i haven't seen any other program to filter AC3 to 4 speakers.
Thanks for answering
KaBeCi

[SKD_Tech]

Try a different version of the AC3Filter

[KaBeCi]

I've just downloaded and tryed AC3filter 0.68 and i got the same problem, i also tryed intervideo decoder and the noise is even louder (may be due to compression)
the noise is a highed pitched noise and is detectable on low audio scenes
any help appreciated
thanks