TL'd th sfx & some dialogue that had been left out. this artist uses some pretty unique sfx; for the large SFX translated here as *plap* or *whap*, I'm actually not 100% sure if the second character is supposed to be キ(ki) or 干(hi). However both make a very similar sound, so the SFX would basically be the same. I guess it could also be 千(sen), but that would be weird and not make sense in this context...

It's ドチュッ. It seems to be an onomatopoeia that evokes the hard slamming and wateriness of insertion.