Theories?

I am hearing things. Unexplainable things. Here's the set-up:

Our kitchen is downstairs, as most kitchens are, and Kate's bedroom is right above it. The girls' bathroom is also above the kitchen, a little bit towards the middle, and probably above the cook top and microwave. Kate's bedroom is carpeted; the bathroom is tile. A few times a day for at least the past week, I randomly hear what sounds like someone dragging a heavy object across a solid surface floor. Specifically, it sounds like a bed or large table grinding across a tile or hardwood floor. This happens when there is either no one upstairs, or when the girls are asleep. In any case, there is no piece of furniture that either of them could be dragging around to make the sound.

It's not really creeping me out; it's just weird, and I can't explain it. In the vicinity are:

• a refrigerator (with ice maker, but we rarely use it, so it rarely makes ice, and even the making of the ice does not sound like what I keep hearing)
• the laundry room (neither the washer nor the dryer are in use when it happens)
• a water heater (in a cubby hole in the garage, right next to the laundry room. This is the only thing that is a possibility in my mind, although I can't see how it could be making this noise.).

Any ideas? Is our house haunted? It seems a little new for this. It is intriguing, however.