Strange Noise when importing an FLV file with Alpha Channel

Hi Guys,

I have a strange issue with the FLV files i imported to my site.
These FLVs were imported with Alpha channels.
In my site there is a button, that when its pressed for each FLV it starts playing.

The Problem is after its been playing for awhile there is a strange Noise at the background that starts appearing slowly. (Please see attached file).

The only code i have on each FLV is for transparety and a Loop (Plaese see below):

Anyone knows maybe what is this thing? i never saw this before.

Thanks For your help

[COLOR=red]onClipEvent (load) {
this._alpha = 0;
this.dir = -1;
onClipEvent (enterFrame) {
if (Math.round(this._alpha) == 0 || Math.round(this._alpha) == 100) {
this.dir = -1;// change the direction of the fade
this._alpha += 5


on (complete) {
this.autoRewind = true;;