The Rat-a-Tat Mystery. Enid blyton

Staying at a creepy old house in the country surrounded by snowed-up roads, cut off from civilisation, Roger, Diana, Snubby and Barney witness strange goings-on, all tied up with ancient old legends of how the great knocker on the front door always bangs by itself to warn that traitors are in the house. When someone bangs on the door in the middle of the night but then mysteriously vanishes, faces peep in at windows, snowmen seemingly move by themselves and the phone is suddenly cut off, the children decide that it's their job to unravel the mysteries surrounding Rat-a-Tat House.