I hear noise in all the channels
When you hear noise while watching TV, you have to perform a Sound Test to see if the test sound is played clearly.
If there is no noise during Sound Test, then it may due to poor reception on the TV or poor connection of peripheral (Set-top box, game console, DVD Player, Blu-ray Player, etc.) You may have to check the connection.
If the noise only occurs on a particular channel, it is possibly caused by unstable reception or an issue with the broadcaster.
How to fix
Smart TV ‘12~’13
[Smart] or [Settings] → [Setting] → [Support] → [Sound Test]
WebOS TV after ‘14
[Home] →[Settings]→ [All Settings] → [Sound] → [Sound Test]