"Flap" sounds like something (a flag, a sail) flapping, and "flop" is the dull sound of something falling down flat.

而flop听着就象什么东西直直落地发出的沉闷一响.

youdao

Look, whoever is making that flapping sound, quiet down.

谁在弄出那翅膀拍打的声音？静下来！

youdao

Manifestation: Accompanied by the sound of flapping wings and cawing crows, Astaroth manifests as a hideously ugly angel.

显现：伴随着一阵扑翼声和乌鸦的鸣叫。

youdao

Along with the flapping, the robotic bird included a small sound generating system to play authentic calls and swamp noises.

ENGADGET: Alt-week 3.2.13: A mission to Mars, robosparrow and facial recognition in fertility treatment

While the project might sound a little bit gruesome, the research helped gather data to support the hypothesis that wing flapping is often used to deter rivals.

ENGADGET: Alt-week 3.2.13: A mission to Mars, robosparrow and facial recognition in fertility treatment