If your vehicle is showing high-pitched squealing noise when starting or accelerating, here are the most likely causes and the parts you'll need to fix each one.
The rubber belt that drives engine accessories hardens, glazes over, or stretches over time. This causes it to slip over the metal pulleys and squeal under the heavy load of starting or accelerating.
The internal spring of the automatic tensioner loses its strength over time. It fails to keep the belt tight enough against the pulleys during rapid engine acceleration, resulting in belt slip.
Spilled oil from a messy oil change, or a slow coolant leak, coats the drive belt. This drastically reduces friction and causes the belt to slip and squeal even if it is relatively new.
The bearings inside the accessory pulleys dry out and wear down. As engine speed increases, the failing metal bearings emit a high-pitched squeal that mimics a slipping belt.
The alternator works hardest right after starting the car to recharge the battery. A failing internal bearing will squeal under this heavy electrical and mechanical load.
On turbocharged vehicles, a failing compressor bearing or a leak in the pressurized intake tract can cause a high-pitched squeal or whistle specifically during acceleration.