I think I can solve that for you. Obviously an elf walking by, fell into the soundhole and is humming pleasantly to gently let you know he's there inside, trapped but not panicking, at least...not yet.
To remove pesky elves from your uke, speak into the soundhole saying "Oma oma onomatopoeia" which is Elvish for "Help is on the way! Stay strong, my friend." Then look for his tiny hands hanging onto the rosette. Now turn over your uke and shake, but gently, gently, my friend. The elf will drop out (it is kind to have a velvet pillow handy as a landing pad) and he will thank you by farting, which will have sparkles, smell like roses and will make you cry when it's over.