how a ”noun clause” should be?

A noun clause is a group of words containing a noun or a pronoun that function together as a noun or pronoun, as the subject or object of a verb of a sentence.

Noun clauses are often preceded by an introductory word or expression such as 'That' or 'The fact that'. This construction is always followed by a singular verb. It is generally used only in formal written English (e.g. That the problems are immense is obvious). In spoken English it is more common to find an anticipatory 'it' construction (It is obvious that the problems are immense).
A second noun clause may be used to define or explain the first in any position in the sentence. In this case, the second noun clause is always separated off by commas. This construction may be replaced by a relative clause.
Example : The people that I saw coming in the building at nine o'clock have just left.
('The people ... nine o'clock' is a lengthy noun clause, but it functions as the subject of the main verb “have just left”.)

he pretend not to hear me – to be frank I do not think “not to hear me” is a noun clause, since it does not serve as a noun nor pronoun. The whole phrase “he pretend not to hear me” can be classified as “relative clause”.

A noun clause is a group of words containing a noun or a pronoun that function together as a noun or pronoun, as the subject or object of a verb of a sentence.

Noun clauses are often preceded by an introductory word or expression such as 'That' or 'The fact that'. This construction is always followed by a singular verb. It is generally used only in formal written English (e.g. That the problems are immense is obvious). In spoken English it is more common to find an anticipatory 'it' construction (It is obvious that the problems are immense).

A second noun clause may be used to define or explain the first in any position in the sentence. In this case, the second noun clause is always separated off by commas. This construction may be replaced by a relative clause.