The injector lines all should make a "thud" sensation when you touch them. They'll vary with how much space exists between bends and supported areas. The thumping on the line is from the injector pintle opening and seating.

Nailing is very audible. You'll hear it coming from the top end of the engine and it's unmistakable. Your video does not seem to show the noise coming from anywhere around the injectors or the top end, nor does it sound like the typical nailing. The loudness of the noise from underneath the car suggests bottom end. Additionally, nailing typically doesn't occur as RPM's drop after a throttle blip since fuel should not be getting injected at that point.