How does engine noise change during descent / approach?
It is important to notion that an aircraft does not provide a uniform sound but features several sources of noise. The noise generated by the individual sources has separate characteristics and is transmitted into different directions.
For example, the sound of the engine front fan is sent into the direction of flight, creating a cone. The sound of exhaust jet and low-pressure turbine is transmitted backwards. Passive sound sources sent according to their positioning on the aircraft. Underside structures transmit downwards.
When you experience an aircraft transition overhead, the sound created by the aircraft may be steady but you experience the sound of changing sources.