I'm guessing that when Shazam is 'listening', it's converting the sound to ones and zeros, and comparing them to its database.

When on holiday there was this band having a go at 'Sultans of Swing', and I tried it out. But Shazam was having none of it. Probably didn't help that the singer seemed to be just mouthing the sound of the lyrics with no understanding of what he was 'saying'.

Perhaps Shazam was just saying to me that it wasn't going to dignify that racket with a reply.