Maybe you have a mouse...hopefully not and I'm sure you would have noticed. Is there something you moved under the bed that wasn't there before? Today, for example, I had moved these boxes by the front door, that Benji saw me moving, and then I went up stairs and he followed and a couple minutes later I heard him growling. So I went to see what he was growling at and he was staring down the stairs where the boxes were. He's weird like that he growls at everything.