noise occurs if combined with complex sound

At first I used matlab GUI to design audio-visual display but could
not display the visual part in full screen.
Then I tried psychotoolbox, and the visual display is terrific.
However, because I need to use space-specific auditory cues (convolve
pure tone with head-related transform function, but they should still
be simple matrix for 'sound' command), I got undesired noise in
auditory display.
Did anyone have similar problem? (Or maybe it is only due to my poor
sound card.)
By the way, I use WinXP, psychtoolbox 2.54, matlab 6.5.
Thanks.