He's talking about the fact that when we first see the rex we here BOOM BOOM BOOM! But when he shows up at the end he shows up all Ninja style by us not hearing him coming. Although, with two raptors chasing you down you wouldn't really be noticing the sounds of footsteps or slight tremors because your running for your life.
Johro, good point on that raptor cave. I was thinking the same thing when I read it again recently. It seemed like a very unnecessary risk taken for no other reason then to satisfy their own curiosity. I don't even think they brought weapons.