It's saying I think we're being invaded by woodpeckers when someone knocks on the door.
A riff starts with something real and then spins off somewhere surprising and delightful. In jazz and rock music, it's taking an opportunity between bits of a tune to play something for fun (or to show off).
This is the sort of thing:
If you don't play a musical instrument then you can whistle or sing - or, if you're not musical, just ask yourself a few interesting questions:
What happens if a police car siren gets a sore throat?
What does a dog get out of sniffing lamp posts?
Is it true that clubbing is technically banned under the United Nations Convention Against Torture?
Where you go from there is up to you.
Just, well, riff.
Thing To Do Today: riff. This word appeared in the 1900s and is probably a short form of refrain.