Panasonic Tv optical audio output

Hi everyone i have a 2011 Panasonic TC-P46x3 tv until recently i had no issues with the tv , I decided to get-rid of my comcast set top boxes and buy a OTA Antenna that works find with the internal speakers
and if i use a hmdi input to the tv my stereo sounds normal .except if i watch tv over the air i get a moterboat sound through the stereo there are not settings in the tv to adjust the audio conversion and i tried using a optical to analog rcn splitter that i had and the same problem ps i have a Onkyo TX8160 receiver Thanks Tim