Did I mention that our house was haunted? Apparently it was built on the site of an old pond and is now stalked by the ghost of George, the unpleasant terrapin (according to Son, who is an expert on amphibians from the other side...hang on, are terrapins amphibians?). Apparently one of the reasons he's quite such an unpleasant ghostly terrapin is that nobody ever takes him seriously, which enrages him. He'd like to be scary, but succeeds only in looking like a disgruntled pebble.
Which may not explain things that go bump in the night, but may explain things that sound like an irate slow scuttle.