So, is this a moment where Rainbow Dash kisses Fluttershy? I'm gonna say... nah. Unintentional coincidence.
On the other hoof, that sound seems to be right at the time when Dash's mouth passes by Fluttershy's nose. If it IS supposed to be like that, I guess I can still see this as basically just friendly reassurance. Or something like that.
I own nothing here!
Edit: This video has gotten a heck of a lot more attention than I expected it to. We sure do like analyzing little things like this, don't we?