Uncle Vernon stayed at home again. After burning all the letters, he got out a hammer and nails and boarded up the cracks around the front and back doors so no one could go out. He hummed "Tiptoe Through the Tulips" as he worked, and jumped at small noises.
This is from the book Harry Potter. I don't get what "jumped at small noises" mean in this context. I tried to search "jumped at small noises" and "at small noises", but came out nothing useful.
I guess those small noises refer to what he had hummed? I can picture he was dancing with what he hummed. But I'm not really sure about it.
P.S. [I think he was nervous. He was trying to block those letters sending to Harry.]