Singer's space-filling syllables (3-2)
I believe the answer is:
tra-la
(Other definitions for tra-la that I've seen before include "Sound made when humming a song" , "Ironic; expression of joy or gaiety" , "The flowers that bloom in the spring -- -- (Mikado)" , "Expression of joy or gaiety, musically" , "Joyous refrain in song" .)