Thunderous means

Only then did he breathe a sigh of relief.
It looks like the people should be in this compartment, or even that I might have just rushed out from here, and they would show up!
Now that there is a wall blocking his way, he is a little safer, but he still doesn't dare to breathe too hard. He is afraid that the other party will hear his breathing and identify the position of his head, and may shoot through the wall!
At the same time, he could also feel that there was no sound over there,...