This is often what happens in a band or music class. If the musicians have no had enough time to practice, or have been slacking off, the instructor/teacher might assign them time to nurddle. Nurddle
: the act of playing an instrument aloud without structure, (ie) playing by yourself, not with others. Nurddling
: the act of the nurddle. Nurddler
: the person(s) making the sound.