[VS6] Hiss in MP4's AAC?

[BartBear]

Hi there!

As co-admin (no-profit) and game-cameraman (hobby) of red.orchestra.pl I'm working a lot with games' footage and uploading it to YouTube and here is my problem/question;
For some reason from the 5th version of VS (then I focused on it) AAC made by VS has some irritating hiss distortion mostly on ambient sounds, far distance explosions or gun discharging etc.
It only appears (I hear it) in AAC made by VS; tried to rebuild my system codecs, tried for comparison some other programs which convert raw footage to MP4 and it was fine. That happens nevertheless the bitrate I use (and I prefer to use it high - have no problem with bandwidth so I don't compress much).
I'd switch for mpg, because at high bitrates the video is in perfect quality and sound is perfect as well (I didn't mention the rendering process is much quicker) - but the problem here is YouTube which damage video in its internal re-compression (loosing details like clear small letters etc.) which is not so visible when MP4 has been uploaded.
Have anyone met this problem? What can be done with it?