That's the sound I heard in the bathroom the other day. And no, it wasn't coming from me.
I followed the sound across the room, behind the sink, down to the floor and beyond - out of sight. Fortunately, I took this photo while we were building the bathroom last year:
So I had a pretty good idea what was going on. One of those shiny expensive joints on the left wasn't doing its job. Which meant my job for the day instantly became: Take out the bath, lift the floor, cut the pipes and pull the joints up above floor level, replace the floor and the bath, tighten the joints, buy even more expensive (and lovely because they actually work) push-fit joints and put it all back together again.
It's the kind of unexpected sunny day project that makes writing the book I mentioned in my last post less and less likely (what with building a second guest yurt platform, solar shower, road, sand filter, French drain etc etc before we open again next year).