This post made me so good inside. Such wonderful things to think about. It's true that Justin's whimpering is far more prevalent in fic than on the show, he does in fact whimper (or at least what I would term whimpering), it's not always easy to hear over the soundtrack and Brian's heavy breathing, but it's there sometimes, both in and out of bed. He whimpers upon waking the afternoon he has sex with the computer nerd when Brian grabs his "morning" hard-on, he whimpers when Brian tells him he loves him in 510...and there's many times.
I missed a LOT of his whimpers until I started listening to the show with my earbuds. I watch on my computer most of the time and I miss out on a lot of sounds.
It's also possible it's a semantics thing, which happens so often in online communication. What you might classify as a whimper, might not be someone else would. And obviously someone who fancies the idea of a Justin whimper will be more lenient with its definition. As for myself, what I hear is no way a stretch of the word.
He also has a whine that's a lot huskier and lower and louder than his whimpers. This is one of my favorite of his sounds. One example is the whine he gives when Brian sinks his cock into him after saying, "I want you safe"