Keep in mind that on a 4 holer you need to get enough thrust for takeoff/climb from 3 engines, so each engine only needs to provide 1/3 of the required thrust. On a twin you need to be able to takeoff and climb with only one engine, so the thrust requirement per engine is quite high. The 787-9 has slightly higher thrust engines than the A380, at only a small fraction of the weight (and both are WAY less than the 777). Lower thrust engines, all else being equal, means a quieter ride. The A340 was known for being exceptionally quiet due to having engines only slightly more powerful than a 737.