From the Super Mario Wiki
Bouncing Notes are creatures resembling music notes, which are found in Music Park in Mario Kart 7, Mario Kart 8, and Mario Kart 8 Deluxe. They are found towards the end of the track. They bounce in tune with the melody and, once they land, create waves that lift all racers on their section up a little; a Trick can be performed when this happens. Anyone beneath them when they land, however, will be flattened. Three ticks, similar to the sound of a metronome, can be heard before they land. Bouncing Notes notably bounce faster when the timer is reached, most commonly in the third lap.
In Mario Kart 8 and its port, smaller Bouncing Notes appear near the finish line as spectators of the race.