Shemiras Halashon Halachos
Hilchos Rechilus, chapter 2 , section 1 One may not speak rechilus even before a single person and how much more not before many.
Hilchos Rechilus, chapter 2, section 2 Avak (dust of) Rechilus is also forbidden under all circumstances. If what one says is ambiguous and one says it in a way that makes it obvious that the first speaker meant to denigrate the person one is speaking to, this is certainly forbidden. If one speaks the ambiguous
words neutrally it is not clear whether this is permitted or not. For the listener may decide that the first person certainly meant to denigrate him. Perhaps it depends on whether Reuven, the present speaker would speak this way to Shimon the listener, saying Levi said this of him even if Levi was present before them right now.