It's an interesting question, but I think it might be helpful to first review Kant's view on the phenomenon and the noumenon before giving an answer as to what Sartre does with thought and how that relates to the Kantian position and his denial of the existence of the noumenal or things-in-themselves.
Kant's account of knowledge looks something like this: There are things that we render through the forms of sensibility to get sensibles (=perceptables). Then, we push those through the 12 categories (the categories of the understanding) and have objects that we can know.
This method yields two effects: (1) knowledge of reality and (2) the inaccessibility of the thing-in-itself.
the thing-in-itself is sometimes called by Kant "noumenal."
The phenomenon vs. noumenon language also occurs in the Critique of Pure Reason and in Kant's moral philosophy as a distinction between the determined world where we encounter phenomenon and the noumenal world where our wills are free.
Being and Nothingness is an extremely long book (my copy is around 800 pages), so I can't just guess exactly where you are reading. But my understanding in general would be that Sartre's critique of the "noumenal" is not of the freedom of our wills but of the existence of an objective reality (especially for social objects) that is not a product of our wills. In other words, he denies there are things in themselves that ground our knowledge claims. Thus for Sartre, knowledge claims are also will claims.
There's some complex slight of hand going on in what Sartre is doing. To get a good hold of it, we need to look at Hegel's philosophy which has a very large influence on Sartre's Being and Nothingness. Specifically, I'm thinking of a passage I've published about in an obscure journal on the nature of objects in Hegel. Namely, insofar as objects are things that we know, the nature of objects is for Hegel both grounded in what they are and in our acts of knowing that arrange them into types and give them properties. For instance, "marriage" is a type of social object, because the nature of its existence for a society depends not just on physics but on social understandings of who can relate to whom how and whether such relations are accepted.
Sartre accepts this concept of how social objects work (I assume he's not trying to apply this to physics per se). But he disagrees with Kant about the necessity of the underlying hinge whereby what we are saying is objectively true.
Now we are in a position to talk about "thoughts." For Sartre, thoughts would be part of the activity of our reason, rather than an object for our reason. In other words, they are somewhat veiled but only because they are the method of consciousness rather than its object.
I'm sure if we make them the object, that they are largely what we will them to be. To give a plausible example, if we can read people's mind, we can categorize their thoughts "hate speech" , "anti-governmental", "rebellious" , "sinful". And we do this with other people's utterances.
That's the best I can do without a specific reference.