The air. Slap probably isn’t the best word to describe it but it’s simply the rapid changes in pressure that can occur in the 3 examples above.
The older Hueys are probably the most common or well known example. The popping you hear at a hover is the blades cutting the preceding vortex called blade vortex interaction (example 1). In some flight regimes the descending blade on the retreating side stalls during what’s called retreating blade stall (example 2). The loud crack you hear from approaching Hueys from a distance is from the shockwave that forms on the advancing blade during transonic speeds. (example 3).
Some of your more modern helicopters have minimized (swept tips) the blade slap noise problems. You can still get a pretty good slap in say a Black Hawk but you’d have to be maneuvering pretty aggressive to do so…or descending rapidly and blow through Vne.