It does feel harder after the break and he makes a muffled grunt at the first few strikes before it occurs to him that there's no chance of anyone hearing him here. That's the whole point of this after all. He doesn't have to keep quiet and he imagines Jack will enjoy a more vocal reaction the same way he always enjoys getting him to let that strict self-control of his slide.
The next hit lands and he stops holding back, let's go of that last bit of restraint he was clinging to and cries out, not sure if it's a sound of pain or pleasure and not really caring as long as Jack keeps this up. The bed is wrinkled where his fists have made knots in the sheets and each new strike leaves him shaking and groaning with need, too worked up to do anything but just react.