Is there a word for singing a song substituting meaningless syllables for lyrics?

For example "tra, la, la", "dum, de, dah, dum", etc.

In this example

http://www.youtube.com/watch?v=pQ3VULfbxSc

during his "magic fingers" act, Art Metrano is ______ing the song "Fine and Dandy".

The Wikipedia article on Art Metrano describes it as " constantly humming an inane theme song – 'Fine and Dandy,' an early 1930s composition by Kay Swift."

However, I think of "humming" as a different sound from what he is doing.