waterfall nuisance

Hi everyone, please, from Lolita by Vladimir Nabokov:

Inquisitive parents, in order to pump Lo about me, would suggest her going to a movie with their children. We had some close shaves. The waterfall nuisance pursued me of course in all our caravansaries. But I never realized how wafery their wall substance was until one evening, after I had loved too loudly, a neighbor’s masculine cough filled the pause as clearly as mine would have done;

People begin to suspect about the relationship between Humbert (man in his forties) and Lolita (12 or 13 year old girl). What does “waterfall nuisance” mean in here?

Thanks!

I believe it refers to the sound of a flushing toilet. Earlier in the book—p. 316 in the edition I found—he describes a toilet heard through a hotel wall as sounding like a waterfall.