Update. This is a single handle Moen unit installed in 1990. It pulls out and turns to left and right.
So I went the cartridge route, 28 years old. How could I go wrong.
Bought a cartridge puller at Lowes ($12) and it worked great. The cartridge was a Moen 12801. They don't make that anymore. The replacement cartridge is the Moen 1225. Bought it at Lowes for $15. I installed the new cartridge and it still squealed. Damn!
So I took off the shower head (Peerless multi-function) and replaced it with another shower head from our other bathroom. Squeal is gone. My guess is the shower head has calcium buildup inside the unit.
Note to self: Take the easy route first. The cartridge did seem really worn though. The o'rings were all really flat. It was probably close to its end of life anyway.
Matt30 - Spout do you mean shower head?
Thanks for everyone's help.