Recently my toilet began making high pitched squealing noise after flushing. I have been told it is air in the line, if so how do I fix it. If not what is causing it and how do I fix it? This is the bathroom that is used for all company that visits my home and is a little embarrassing. Please help.

Its not air in the line, it's a loose washer vibrating. Either in the toilet valve itself or the stop valve beneath the toilet.