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Heartbeats. The sound of heartbeats is the first thing he hears that morning, which makes sense because his head is resting against Sherlock's chest and being so close it would be impossible not to hear them. It was the same sound that he fell asleep to the night before.
He estimates it's about 85 beats per minute, Sherlock is a smoker so that's to be expected. Even if his heart rate is a bit high for an adult at rest, William finds that there is no better sound right now. What's more, he could assure without a hint of doubt that it was that pulsation against his ear the thing that made him experience one, if not the best night's sleep he has ever had in his life.
His sleep schedule was always irregular and more than once he spent whole nights awake because of his insomnia. But now, with Sherlock hugging him tightly and holding him against his chest, he thinks he might be able to repeat the event as many times as he wants.
So he accommodates himself in the arms of his lover, because even if it has already dawned he does not want to move from there until Sherlock wakes up and he can say good morning to him properly.
And when he closes his eyes he begins to count the beats, and sleep quickly takes over him again.