I have the exact same issue with my HT set up (Sony HTIB DDW-750P) and the "gurgling water sound" would have been exactly the way I will describe it.

But this sound level is so low that I have to be sitting right next to the speaker to notice it and I kinda never bothered thinking that it must be some thing to do with PL/PLII approximation algo (after all its approximation only, right?)

It happens more in some music CD's, less in others, and its not the test tone for sure....