I think this is a whisper never to be revealed, which is its centrally mysterious point. But it could definitely be a repeat of the original phrase, as eyeboogers pointed out, as that would fit with the unending, cyclical nature of events and Cooper's inability to change them. However, I mostly think we are meant to ponder what horrifying element the whisper may now entail that it didn't before. Cooper is very much shocked by what he heard, and the aftermath. And then that final, silent, post-scream scene, scored to "Dark Space Low" really leaves you feeling some sort of otherworldly emptiness. It's all very effective, for sure.
I have to say that what I'm most fascinated in about this/these scenes in The Return, is, during its first appearance in Part 2, the nearly inaudible whisper that literally says or perhaps even directs the actors (or characters?) to "whisper." I find that to be one of the most uncanny moments in the series. I'm almost certain it's said forwards, not backwards, so how did it get there? I highly doubt that none of the sound-oriented guys, including Lynch and Hurley, noticed that it was left there, so I don't believe it was ultimately by accident, even if it may have started that way. It's basically subliminal...I don't hear it every time I watch it, but it is definitely there, and one of many instances of real-world intrusion and evidence of process adding another layer to what we're seeing/hearing.
I'm surprised no one has tried deepfaking young Sheryl Lee on these scenes...
"The pine weasel escaped"