When I flush the toilet, I hear a whistle in the bowl of the toilet (a loud squeal)

Take oiff the tank lid and flush again. Is the noise actually down inside tha tank? If so it's a fill valve diaphram vibrating. Which type do you have https://www.askmehelpdesk.com/attachments/plumbing/12875d1224099054-toilet-noise-clip_image001.jpg