Hello I've recently received a TV which isn't brand new but as soon as the TV comes on there is a very loud and annoying high pitched squeel, I was wondering if anybody could help me

Sounds like your picture bulb is going to go out. Usually a high pitch like that comes from one of those bulbs getting ready to burn out. Call a local TV repair and ask. Don't just ask one call a few. Good luck.

This sound is generally your power supply going bad. I have seen these continue to run for quite some time, but usually there days are numbered. Depending on size and model it may or may not be worth repair.