Turn It Down!
Inventing a noise doesn't mean you have to personally be able to create the noise. You just have to describe what causes the noise, and you have to give it a name so everyone will recognize and call it by that name the next time they hear it.
For example, the sound of a frog climbing out of the muck in a bog near the fjords in Sweden would be hailed with, "Ahh, did you hear that slartibartfast again?"
(As many as 42 of you will catch the oblique reference there. Probably more, now that I've given it away.)
You can't properly celebrate Invent a Noise Day by simply renaming a sound that already has a name. What would be the point?
This must be an aural sensation that no one has coined a term for before. You can try to create a sound that no one (to the best of your knowledge) has ever made before.
Noises only audible to dogs or other animals don't belong in this category.
And don't get into the tree falling in the forest with no one around schtick. That kind of thing is right out.