There are frequent questions about the A320 noises.

The buzzsaw noise on climbout is a feature of the IAE engined version (as United use), about half the A319/320 built, the CFM engined version does not do this. It comes from the seal around the outside of the main fan on the front of the engine. It is most apparent from the seats alongside the front of the engine. IAE did a retrofit modification to overcome it but it added about 50kg weight per engine for no performance benefit so few have gone for it. It disappears as the aircraft accelerates. It is not a mechanical contact being made but a noise harmonic from the airflow.

The Whee-whee-whee noises you get at engine startup and shutdown are from the hydraulic system. There are two independent hydraulic systems on the A320 powered one from each engine. There is also a Power Transfer Unit which, in the event that the power to one system from its engine fails, pumps pressure from one system to another. When you start, or stop, one engine before the other this cuts in and its valves are what you are hearing. This time it is Airbus who have got fed up with the comments and have redesigned the pump, recent A320s do not do it. It is most apparent from seats over the wing, overhead the pumps. If you ever hear it in flight you have got an engine failure

The noisiest/quietest aircraft is dependent, more than anything else, on where you are seated. Rear-engined aircraft like the MD-80 or the RJs are notably quiet up at the front, the opposite at the back. I have always been slightly disappointed by how noisy the Boeing 777 is in the rear half of the aircraft compared to the similar A330.

Anyone complaining about noise though should have flown in the large 4-engined propeller aircraft of 50 years ago and be seated alongside the props.