Here's a flute rendition of that repeating audio done a while back. =p
https://drive.google.com/open?id=0B_Ozdb8mTFWzdnA5RW9YRVFRSjg
It's hard to notice but it's there in this bit of audio :
https://soundcloud.com/riccardo-sallusti/ua-4th-sound-isolated
Listen, think high pitched almost penny whistle but even higher, and subtle, in the background. Almost on the edge of high-pitched audible.
For me, it just seems like a side-effect of the harmonics (or something, I am not technical with music) that Frontier has added. It doesn't seem like being morse-like to me, if can be interpreted to anything I hear it as a short tune.
Aye, they've been called "inbetween purrs" (just cos they appear inbetween purrs) or interstitial sounds.
People have looked at them but I think the general feeling has been they're just random sounds without any meaning. You might be able to find something though.