Dirt, heat, or possible worn or glazed.
The squeal is actually the brake pad vibrating very fast in it's mount on the cpliper. The real only way to get rid of it is to disassemble the caliper and install a vibration barriers between the caliper and the pad. There are many form of these vibration dampers,,,paper, spray . pastes...etc. The Papers seem to be the most effective in my own experience.
Possible worn out brakes
The high pitched squeal is the built in brake pad warning device telling you the brakes need attention. The grinding noise is what happens when you ignore the squeal warning noise. Have a trusted mechanic service the brakes NOW!
high pitched squeal from left front wheel does not stop when I apply the brakes. It will stop when I turn the steering wheel even a little bit but returns when I straighten out again
Alot of cars have whats called a squealer on the front brakes . . . it makes the brakes squeal when you use them. . . if it doesn't then a good indicator is grinding cause that's telling you that the brake pads have wore down to nothing.
Sounds like low pads. many have built in wear indicators that squeal to warn you.
I would start by tightening the power steering belt if it's separate. If it's one belt alone, tighten or change it if it looks warn. If new breaks squeal it is because the person who changed them did not grease the pads before putting them on.
Automatic transmission fluid does not squeal. You have a different problem. Brakes squeal, bearings squeal, and belts squeal. Fluids do not squeal. You had better take your vehicle to a mechanic.
time for brakes,hurry that metal on metal makes my ears hurt.
At least inspect when you hear squeal. Base further work on what you see.
There are a couple of possibilities. One is lack of lubrication of brake system moving and/or contact points such as between back of pads and caliper housing and/or piston. Another could be that, the rotors/discs are warped/distorted and need to be machined or more preferably, replaced. If you replaced the front brakes (example) and you still have a squeal, it's possible it may be coming from the rear brakes.
There may be damage or misalignment in the front brakes if this is where the sound is coming from. If it is from more generally under the hood, this can be the power steering.
I believe your 4-Runner has an option of either rear drum or rear disc brakes. If the squeal is coming from the rear and you have rear drum brakes, have them cleaned and adjusted. If the squeal comes from the front, a shim may have slipped off of one of the brake pads and is scraping on the hat of the rotor.