"Pop" wrote in message
I'm assuming it's both cold and hot water, mostly the cold, where you see
this, right? If it's just the hot, then there are a few other
possibilities but your descrip sounds like it's not just the hot.

I can hear the bubbling noise when the toilet tank refills, so it has to be
at least the cold. It's happened to SWMBO in the shower, & she usually
turns on the hot first, so I'm guessing it's both.

JF