If your TV has an earphone or audio line out jack, try this to see if it is clear. If so, then your problem is in the final audio amp or speaker(s).
If only one channel of a stereo TV is affected, it is almost certainly the audio amp or speaker for that channel. Interchange connection to the two speakers temporarily and see if the problem moves.
If the problem is at all intermittent - try gently whacking the TV - then it is likely a bad connection - either a cold solder joint or a dirty or tired IC socket.
The audio amplifiers in newer TVs are almost always ICs and replacements are usually readily available. If the IC is in a socket, remove the IC, clean the pins and socket contacts and reinstall it. Sometimes, the contacts on old socket lose their springiness and do not provide solid connections. Such a socket will need to be replaced.
If the set uses discrete transistors, it s also possible for one of these to become noisy.
If your TV is fairly old - 10 years or so - this may be an alignment problem requiring tweaking of a coil in the sound IF. See your service manual. It may be possible to have similar problems with newer TVs but this is relatively rare.
There could also be bad electrolytic capacitors, probably in the power supply area. Even though you might think this would result in hum and there is none (even when there is no audio in the program or the sound is turned down) dried up caps can result in distorted sound that may sound like a sort of clipping. An ESR meter is best for testing (with power off!) but carefully jumpering known good caps across suspect ones (again with power off, then turn on the set and check), will eventually find the bad one(s).