You would hear the usual white noise between stations, just by virtue of FM reception relying on high IF gain ahead of a limiting stage. But judging from the description of the block diagram, I suspect the noise you'd hear when tuning in a station would be very unfamiliar, maybe a lot of distorted tones and whistles. I can't believe they'd have gone to so much trouble to hide it otherwise.

It's a much more interesting chip than it initially appeared to be, that's for sure.