It is difficult to answer this question definitively. This is because Conrad himself never specifies exactly what things were "whispered" to Kurtz. All that we can do is infer what the narrator thought that Marlow thought. That is the beauty of this novela's ambiguity.
That said, we can make an inference based on the fact that Marlow makes this observation about Kurtz after witnessing the shrunken heads that Kurtz had outside his house. According to Marlow, the heads demonstrate "that Mr. Kurtz lacked restraint in the gratification of his various lusts..." So we can infer that the wilderness awakened in Kurtz a basic, primitive bloodlust that undermined his connection with the civilized world that he professed to champion. It "whispered" to him that he was capable of things that he never would have thought possible before.
Marlow later points out that the wilderness had a spell that
"seemed to draw [Kurtz] into its pitiless breast by the awakening of forgotten and brutal instincts, by the memory of gratified and brutal passions."
This spell caused Marlow to feel that Kurtz
"had kicked himself loose of the earth...he had kicked the earth to pieces."
So perhaps Marlow feels that Kurtz' connection with the wilderness had given him some sort of transcendent, earth shattering revelation.