sound problem - Distorted Audio Chattering sound?

i purchased a 46 in TOSHIBA REGZA tv model# 46xv645u serial# 946433t59239k1 and i am having a chattering distorted audio sound after the tv has been on for awhile< time varies. i purchased a 3 year service contract when i purchased from best buy. the geek squad has been to service 2 times and the cable co. three times the only answers that i can get is it could be, but my problem still exist. service agreement expires 12/27/2012 need help!!!