Justice, according to Aquinas, divides into two kinds: distributive and commutative. Commutative justice deals with exchanges of some kind, and itself divides into two, according to whether the exchange in question is involuntary or voluntary. The vices that correspond to commutative justice are injuries against another person, which can be done by deed or by word. If the injury is verbal, it can be a verbal injury accomplished using the means of justice, i.e., the courts, or can be extrajudicial. It is in this last category, extrajudicial verbal commutative injustices against the will of another, that our vice today comes. Aquinas discusses five such vices. It's somewhat remarkable that such a precise category has five distinct categories devoted to it, but as it happens these vices -- contumelia, detractio, susurratio, derisio, maledicere -- are serious problems, being easy to acquire (they require no more than the ability to speak) and capable of doing some serious damage. Contumely is deliberately speaking ill of present people in order to ruin their good name; detraction is deliberately speaking ill of absent people in order to ruin their good name.
Susurration literally means "whispering" (it is onomatopoeic, of course, so sounds like what it means), and while it's not hugely common, we use the same word in the same way at times in English, as in the phrase "whispering campaign". Aquinas also says that the whisperer is bilingual, which he means not in our figurative sense but in the literal sense: the whisperer is two-tongued. The whisperer speaks ill of another person secretly. In this, it is very much like detraction, and the two can be easily confused. When we are talking about verbal injuries, the injury does not lie in the sound but in the sign, and signs are governed by the particular intention or aim involved in using them. The difference of intention here is that the detractor is out precisely to speak ill against another person -- that's the whole point, attack someone in secret by speaking ill of them. The susurrator is actually not interested in any direct attack, even in secret. The goal of the susurrator is to turn people against someone. As Aquinas puts it (ST 2-2.74.1), the whisperer "intendit amicitiam separare", is aiming at cutting off friendship. Whisperers isolate people against their wills by giving their friends reasons to withdraw friendship. In principle, the whisperer doesn't actually care whether it's by speaking ill or not, and one can imagine a whispering campaign that proceeded entirely by speaking well of them. Actually, I was just last night watching an episode of Yes, Prime Minister!, in which Sir Humphrey Appleby, a master of susurration, prevents someone from getting appointed as Director of the Bank of England not by panning him but by enthusiastically complimenting him on things that make the Prime Minister wonder if he is really good for the appointment at all. As Sir Humphrey puts it, you have to really get behind someone before you can stab him in the back. The whisperer, in other words, is only incidentally interested in speaking ill; what he really wants is to say something about another that others will find unpleasant, even if it is in reality good.
When we are dealing with sins against other people, the worst sins are those that harm them most, and as we are only capable of harming people by depriving them of some good, harm is determined according to the greatness of the good lost. Therefore the habit of whispering is a worse vice even than contumely and detraction -- reviling and backbiting, as they are often also translated -- because friendship is an extraordinarily great good. We cannot live without friendship; even the most barren life is only possible through at least some minimal kind of civic friendship that make life and survival possible. Contumely and detraction are both concerned with reputation; but reputation is a secondary good, primarily of importance because it facilitates friendship. As Aquinas says, "fama est dispositio ad amicitiam, et infamia ad inimicitiam" (reputation is a disposition to friendship, and bad reputation -- infamy -- to enmity). Contumely and detraction take away honor from others, but susurration takes away the love of others. It is the poisoning of other people's minds against a person, and thus, despite the fact that you can in principle do it entirely by saying only good things about a person, it is a more severe violation of love of God and neighbor, which is the ultimate standard of the grievousness of a vice in Aquinas's account.
St. Gregory the Great in the Moralia (Moral. xxxi, 45) lists susurration as one of the daughters of envy. Daughter-vices are vices that a capital vice naturally tends to create in those who have it. When Aquinas he talks about envy, he accepts this list, and explains it, and thus susurration's place on it, in this way. Envy covers a process, with beginning, middle, and end. I suppose you could say it's a vice with a narrative structure. At the beginning, one tries to lower someone's value in some way, and this can be done secretly (which is susurration) or openly (which is detraction). To the extent that one is successful, this leads to exultation at another's misfortune, and to the extent that it is unsuccessful, it leads to affliction from their prosperity. And the end of it all is simple hatred. We do have to be careful, though; these are Gregory's terms, and Aquinas is merely explaining. It's possible to argue that the 'susurration' mentioned by Gregory is actually Aquinas's 'detraction', and the 'detraction' mentioned by Gregory is actually Aquinas's 'contumelia'. The text is not precise enough to give a definite answer on the question.
You'll notice, incidentally, that with this vice rather than others I have no classical sources. There are, as far as I am aware, no significant classical discussions of the vices of verbal injury, although scattered identifiable references come up (particularly for contumelia); and there are certainly none of significant influence. Virtually all of Aquinas's references, for instance, are to the Bible, interpreted through the Latin Fathers of the Church (Augustine, Isidore, and Gregory especially). The emphasis on the sheer grievousness of vicious speech is something Christian philosophers and theologians inherited from the Jewish roots of Christianity, and one finds the same emphasis in rabbinical discussions. If you look at the books of Torah, you notice that there's a lot about leprosy, and one might ask why this occupies such an extraordinarily large space. The medieval rabbinical answer was that leprosy, in the Biblical sense, is associated with speaking evil of others. Leprosy, the visible blotting of oneself, is a fitting punishment for evil-speaking, the attempt to blot another invisibly; the one is actually in some ways a fairly decent analogy for the other; and when God is portrayed as actually striking someone with leprosy throughout the Tenach, it tends to be for evil-speaking. Everything the Bible says about leprosy, then, can be read as an indirect comment on evil-speaking. There is a Jewish proverb that when one speaks ill of another, it is as if one denied God. Christians and Jews both share proverbial associations of evil-speaking as a sort of verbal attempt to murder. The three most terrible sins recognized by the rabbis were idolatry, sexual perversion, and murder, and Maimonides (Hilchot De'ot 7:3) says, flat out, that evil-speaking is equivalent to all three. And one has only to look at how they talk about evil-speaking, lashon horah, that what Aquinas describes as detraction and susurration are front and center in their understanding of it.
When Aristotle talks about vices of speech, it's clear that he's mostly talking about social gracelessness. The moral weight of speech, the recognition of serious evils of speech, is something that in the West almost entirely derives from Jewish thought.