He heard the hum as well as the steady calm beating of her heart given how close he was. Knowing he could bring her such peace and tranquility was a reward in of itself but a dragon was also greedy so that proved hardly enough. So he moved his washcloth over Lucy's back meticulously making sure to be thorough but also quick. Since he still had her front to do and he would have to move fast before she caught onto his sneaky intent.