http://www.exhaustsoundclips.com/sound/ford_mustanggt_2003_4.6v8_flowmaster_american_thunder_sn009_.mp3
I found a clip...not very good quality, but it will give you an idea
Found another. all stock. Its a good one.
http://www.mustangexhaust.com/sounds/99GTFlowmaster2chamber,stock,stock.mp3