A grating noise when accelerating a car is often a sign of bad brakes. The disk brake pads rest against the rotors at all times. When the material on the pad wears through, the metal plate that backs the pad rests against the rotor and makes the noise.
When metal rubs against metal, it quickly damages the disk brake rotors. The noise and damage increase when the driver waits to replace the brakes. When the vehicle needs to have the brake pads replaced, the mechanic is likely to recommend the driver have the rotors turned or replaced as well, depending on the amount of damage that the condition has caused.
Drivers use their brakes several times each mile they drive. With worn brakes, it is sometimes impossible to stop a vehicle safely. Most brake pad manufacturers add a wear indicator in the brake pads. The indicator is a small piece of metal that makes a high pitch squeal when the pads reach a certain wear point. When brakes begin to make this noise, the driver should have them inspected and replaced if necessary. Replacing brakes in a timely manner reduces the cost of repairs and ensures the vehicle is mechanically ready to stop in an emergency.