belt slipping??????

I just had my tranny rebuilt. Now I notice when I floor it from a stop and when it shifts into second I hear a swoosh sound of some kind as it go's into second. This also happened before the rebuild. I had worn bands and clutches. If I'm at 1/2 or 3/4 throttle it shifts aggressive but with no swoosh unlike when I'm at WOT going into second .I don't thinks its the tires breaking loose because its not high pitched. Could this be the 2 belts making the noise. Or i'm thing it could be the tranny. Has anyone heard this sound before.I have a new gatorback main belt,but the ac gatorback belt is back ordered and I should have them both on by tuesday night. I'm just afraid that its something in the tranny. And I don't think the rear end would make that sound......
2002 z28 A4 76,000 miles ..
Please any info and help please. Before I take it back to the tranny shop thsi week