Why did the Navi find it necessary to reveal thru Chazal that Manoach was an Am Haretz.
Lchorah its not part of the entire narrative, why do we need to disparage him for eternity?
If you're asking that it appears to be Lashon Hara if there is no need for it, Chazal will answer that they derived important lessons from this, since it will help explain how Manoach behaved towards his wife and his son Shimshon and teaches us how to behave properly.
Technically speaking there is also no prohibition on speaking Lashon Hara regarding a dead person except for a special Cherem (excommunication) mentioned in the Midrash Tanchuma (Va'etchanan 6) and this is only regarding Motzi Shem Ra meaning speaking lies about the dead person but not if one speaks the truth. Even though true Lashon Hara is also wrong, if one is not stating a new fact about the dead but mentioning what was already known about his actions, it would be permitted (Nimukei Orach Chaim 606:1 (Munkatch) , Chut Hashani Yom Hakippurim p.106.) In this case presumably what Chazal said was known beforehand about Manoach so it does not constitute Lashon Hara.
I was coming from a point, of "Vos felt unz ois dos tzu vissen"?