The shower is definitely producing a fair amount of white noise to mask his moans. Sherlock's senses being particularly sharp, he can probably still make out that there's some degree of sex going on in the shower. He's not going to say anything about it, though, and will move to another part of the apartment to not listen.

Joan isn't thinking about that, though. His moans are amazing, as is the hand in her hair. When he lets go of her hair she reaches up to put her hand over his, holding it to her head as she continues to slowly move her mouth along his cock. He can tug at her hair. It's okay. She likes it.