Brakes

Best is to have the brake system inspected at a repair shop. Most commonly by the time the pads are worn enough to start grinding metal-on-metal, the rotors will under their legal thickness and will have to be replaced too. Those are usually pretty inexpensive.

If you want to get more involved, there is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

A sticking caliper can cause very rapid pad wear. A ridge of rust on the outer edge of a rotor can cause a harmless but annoying grinding noise. A rock stuck on the friction surface of a pad can cause a grinding noise. A bent splash shield can cause a noise that may only occur during cornering. Have the brake system inspected. The people at tire and alignment shops are experts at finding the causes of noises, vibrations, and bad tire wear.

Was the previous brake job done by a professional or a do-it-yourselfer?

Reason I asked is there is a lot more to a brake job than just hanging new pads. There's a lot of things professionals do to prevent noises from occurring, and there's a lot of things do-it-yourselfers can do to accidentally cause noises. Knowing who did the last job will help in knowing where to start looking.

A squeal can result if the edges of the new pads weren't prepared before they were installed. The fingers on the calipers that contact the outer pads, and the surfaces of the pistons that contact the inner pads have to be cleaned with a flat file to remove any rust, dirt, or glue, then those surfaces must be coated with a special high-temperature brake grease. The calipers' mounting pads must be cleaned and coated too so any squeal that does occur doesn't get transmitted to the steering knuckle where it will be amplified.

There are also multiple things to look for when machining the rotors or when installing new ones. One thing do-it-yourselfers are often guilty of is getting grease, including fingerprint grease, or other contaminants on the friction surfaces. Once the pads and rotors have gone through a heat-up cycle, those contaminants will soak into the pads and the rotors. Cast iron is porous and will absorb grease. That will cause a squeal. The only way to solve that is to replace the pads and rotors again.