The answer here How and why is Subaru connected to the Witch (Satella)? hints that it might be "I love you", and that communication with Satella becomes clearer the more Subaru dies (and more loops trigger, bringing his timeline/world closer to Satella's). [Citation needed]
What does Satella tell Subaru as he spams declarations of his ability? I was wondering if she generally says the same thing each time (so removing the case when Subaru took Betelgeuse along for the ride), if it was intentionally muted in the anime in previous instances. Does the LN/WN have her silently whisper?