I have an upstairs bath creating a dripping noise in the wall which is baffling me. There is one scenario (only one been able to identify so far) which creates the drip noise. Shower/tub water is turned on, and the spout diverter is pulled up to initiate the shower. Then the toilet in the same bath is used (shower still running). I hear a drip - drip - drip sound, lasting maybe 20 seconds or so, then stops. No signs of water leaking out in wall or ceiling (although we have had problems with the bathtub overflow leaking when in bathtub overfills - not the situation here). I can hear the sound clearly at the 90deg bend of the PVC pipe in the basement pretty much underneath the 2nd story bath. In fact the sound there seems to be drip-drip-drip, then eventually a gush of water, then eventually I can feel the pipe warm up as the hot shower water warms it. The pipe and area around the pipe in the basement seems dry.

Any ideas on the cause or things to check? This just started a few weeks ago. Would a partly clog pipe cause this? The drip sound always stops well before the shower is finished. Thank you.