VOICELESS PHARYNGEALIZED ALVEOLAR SIBILANT [sˁ]
Tiberian Hebrew ṣade sounds like English "s" pronounced emphatically. For those familiar with Arabic, it is pronounced like the letter ṣāḍ (ص). To pronounce this letter, constrict your throat and make your tongue concave to create an open cavity in the mouth while pronouncing "s".
The fourth place of articulation is the teeth, from which are heard four letters, namely זסצש. In this place of articulation the letters are not divided into light and heavy as in the preceding places of articulation.
Translation from Khan (TPTBH II.L.1.3.8)
ואלמחל אלראבע הו <אלאסנאן> יסמע מנה ד̇ חרוף והי ז̇ס̇צ̇ש̇ והדא אלמחל לא ינקסם פיה אלחרף אלי כפה ותקל מתל אלמחאל אלדי תקדמתה
Text from Khan (TPTBH II.L.1.3.8)
EMPHATIC VOICED ALVEOLAR SIBILANT [zˁ]
The medieval grammarians also talk about a secondary pronunciation of ṣade, which sounded like an emphatic/pharyngealized version of zay. For those familiar with Arabic, it is pronounced like the letter ẓā (ظ) in some pronunciation traditions of Arabic. To pronounce this, constrict your throat and make your tongue concave to create an open cavity in the mouth while pronouncing "z". It is not entirely clear when such a pronunciation was used, but there is at least a specific mention that it was used in the name אֲמַצְיָהוּ 'Amaziah'.
In fact, there seems to be a degree of confusion regarding the specific terminology for this sound. Indeed, the term zāy makrūkh (see quotations below) may actually refer to this voiced pronunciation of ṣade.
It is said that there are some who attribute a particular feature to zayin and call it zāy makrūkh. I have not, however, been able to identify their purpose in using the term makrūkh, in order for me to be able to describe it, as I shall describe the purpose of the Tiberians (in the use this term makrūkh) with regard to the resh.
Translation from Khan (TPTBH II.L.1.2)
וקד קיל אן תם מן יגעל ללזאי חכם ויסמיה זאי מכרוך ומא וקפת להם עלי גרץ̇ פי קולהם מכרוך פכנת אדכרה כמא אדכר גרץ̇ אלטבראניין פי אלריש
Text from Khan (TPTBH II.L.1.2)
It has been stated previously that I do not know anything that I can report about the zāy makrūkh. I only mentioned it so that it be known that letters have different attributes and because speech is dependent on letters.
Translation from Khan (TPTBH II.L.1.9.8)
וקד תקדם אלקול אן אלזאי אלמכרוך מא ערפת פיה שיא אדכרה ואנמא דכרתה ליערף אן ללחרוף אחכאם ולאן אלכלאם מתעלק באלחרוף
Text from Khan (TPTBH II.L.1.9.8)
SOURCES AND FURTHER READING
Khan, Geoffrey. 2020. The Tiberian Pronunciation Tradition of Biblical Hebrew. Cambridge: Cambridge and Open Book Publishers. §I.1.7, §I.1.18.