Oh, Sam knows. When he sorted it out, even he's not entirely certain, but he did and it's something he loves to do for Castiel, too. To let him know that he's good, that he is worthy and loved.

Those noises and vibration, though, go straight through Sam. He keeps his strokes steady, but it isn't long before his fingers are curling in Castiel's hair and pushing deeper into his mouth before he groans, pulling back as he comes so that Castiel gets the taste of him in his mouth, just as he'd said he wanted.