Loud squealing sound, only while driving (2002 Toyota 4Runner)

Question

Wondering if someone can help me eliminate potential issues.

• Squealing, somewhat screechy, somewhat chirpy noise.
• No sound at all while parked, even reving up to 4,000 RPM.
• Sound starts while moving (any speed), by hitting the brakes.
• Reminds me of the sound 18-wheelers make in New York City in stop-and-go traffic.
• Sound persists even after completely letting go of brakes, though softer.
• The higher-pitched "chirps" sync with the speed of the wheels.
• Pitch very closely coupled with brake pressure: I can practically play a tune with my foot.
• Tried driving 15mph on empty road, killed ignition, letting car roll, hit brakes. Sound is there.
• I feel like I distinctly hear it on my side, but I don't have a passenger to help me listen from the other side. I hear it very loud on the highway when in the fast lane next to concrete dividers.

The serpentine belts would make a sound even while idling, no? Would brakepads make noise even while brakes are disengaged?

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Additional Information

Car is a 2002 Toyota 4Runner. I just completed a trip across the U.S. My roommate's a hobby mechanic, so he did the following work a few days before my trip:

• Oil change and oil filter.
• Replaced all 3 serpentine belts.
• Checked and filled tire pressure.
• Changed air filter.
• Changed fuel filter.
• Topped off coolant.
• Topped off washer fluid and replaced wipers.
• Topped off brake fluid.

He also did the following within a few months of the trip:

• Changed spark plugs.
• New battery.
• New tires, alignment.
• Transmission fluid and filter.
• Differential fluid.

Answer 1

Sounds exactly like worn brake pads to me. Have you checked those recently? Brake pads actually have a wear tab on them like the one pictured below. This is designed to make noise (that chirping sound) before the brake pads are catastrophically low. The noise would be present almost always while driving as the tab is meant to be in constant contact with the brake rotors once the pads get low enough. I would definitely start by looking at the condition of your brake pads. While you're down there, if the pads look fine, then I would look for something stuck and rubbing against your rotors. Sometimes the rear shield behind the rotor will get bent and rub against it. Good luck!

Answer 2

get the brake pads checked with brake disc. when any water or dust particles comes in contact between disk and pad then the noise starts. cleaning of pads by polish paper may eliminate the noise or replace brake pads. Sometimes material from the brake pad came out and starts rubbing on the disc but this noise goes after certain period when the material wears out. If service is done recently then brake pads might have checked by technician as a routine check up so check if you have got any report from him. Its brake so get it checked as soon as possible.