Sounds like a belt slipping. They make screeching sounds. Next time your there start it up, run it untill you smell the burning,turn it off then feel each belt & pulley. Be careful not to burn your self.
Just had a thought, if you cant be arsed to change the oil then your never going to change a belt are you? Sod it just drive it untill it dies a slipping belt is nothing to worry about on a old knacker.