Q: I just got my serpentine belt replaced and it's squealing when I start the car and sometimes when I hit the gas

Noise of that type is due to a failed or failing bearing in one of the pulleys or accessories (e.g., alternator, water pump, A/C compressor) that is driven by the serpentine belt, the belt is not properly tensioned, or the rotating pulleys driven by the belt are not all in the same plane. I would first check to make sure that all of the pulleys are aligned. It is not unusual, for the pulley on the crankshaft harmonic balancer pulley to move (generally outward) relative to its original position. The pulley is rubber bonded and over time can move. If that pulley moves, that means that the belt groves are no longer running in a perfect plane and that alone can cause noise. With regard to the various ball bearing (and greased) mounted pulleys and accessory pulleys, the grease in those bearings eventually dries out and that can contribute to or cause noise of the type you are describing. If you request serpentine belt inspection - noise diagnostic a certified Mechanic, dispatched by YourMechanic right to your location, will identify the origin of the noise that you are hearing and thence get the issue resolved for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.