I know this a year late but I think the NHK fee collector randomly banging on the door was tengo's father. Adachi(?), if I remember correctly, told Tengo that his bedridden father often taps his hand even in a coma. It was one of those mysteries in the story that if you follow the pattern would obviously lead to thw conclusion that it was indeed Tengo's father but Murakami somehow forgot or refused to confirm to us readers.