I have taken some flying lessons and I think you will often hear the engines spooling up and down on final because they are adjusting altitude, and they might be coming in too low or high so they have to adjust accordingly.

Also flaps and gear are extended in the final because the glidepath is quite shallow which cause more drag, so to keep the airspeed where it should be and not higher (for safety) the engines need to put out more thrust, thereby causing the extra noise heard on final. In the initial descent, you probably are gliding with the engines at idle causing more drag than thrust. It is the most efficient way to descend in a jet.

I think you will often hear the engines spooling up and down on final because they are adjusting altitude, and they might be coming in too low or high so they have to adjust accordingly.

Also flaps and gear are extended in the final because the glidepath is quite shallow which cause more drag, so to keep the airspeed where it should be and not higher (for safety) the engines need to put out more thrust, thereby causing the extra noise heard on final. In the initial descent, you probably are gliding with the engines at idle causing more drag than thrust. It is the most efficient way to descend in a jet.