2014 Toyota Highlander - Noisy defroster

Car makes a loud noise when A/C and Heat is on. It’s like a loud whistling noise and it continues off and on while driving. Very annoying, both Heat and A/C works fine.

Sounds like you mean when AC and defrosters are on. At any rate, my first reaction would be that your AC compressor needs to be looked at by an AC Shop.

Does the noise vary with fan speed? You might have something in on of the vents.