All through the movie, whenever the Spinosaur is approaching his foot steps precede him just like the T-rex in the other movies. In the scene at the fence the Spinosaur would have walked right up to them if the phone he swallowed had not rang! Was he tip-toeing?

Also the T-rex and the Spinosaur were not the largest dinosaurs but they were the only ones to make that thud sound as they approached.