A friend recently asked me: 'What do you call an arbitrary word / sound that replaces another word / concept (thingamajig, watchamacallit and gizmo, or words found often in pop songs, 'my ding-a-ling, my ding-a-ling, I want you to play with my ding-a-ling', or words like bling)'

I am unable to answer him - any help? I am not certain that the examples he gives all fall within the same category, but a lead on this would be gratefully received.