I hope not.
Back to the topic, it is the same sound we hear when when Urianger falls in the water in 5.2 . So I just assumed it was a sign that the manifestation of them on the First was nearing its limit, which was a big deal in 5.3.
I just thought Y'shtola's speech to Runar was a good intentioned lie, or just a way for her to stress how she is attached to him... I never thought more about it really.
But I wouldn't be surprised if Y'shtola found a way back to the First actually. Her character hasn't seen major plot relevance for a while and I wonder if 6.0 will either bring her back in the spotlight or if it could be a way to give her some kind of quiet retirement, given that her days are counted and that she has escaped death 3 times I believe.