Clicking/tapping noise in bathroom what is it?

[Mitsi10]

Been in my private let house for 8 month now (not going so well) I'm now noticing a strange clicking /tapping noise coming noise coming from the bathroom ceiling noticed it over the past week or so usually after I pull the plug on the bathroom sink or bath but this after noon I heard it again every couple of seconds but wasn't using sink or bath?? Any ideas on what this is

it is probably the plastic waste pipes expanding as they heat up when hot water goes down them, then contracting later as they cool down.

It is possible to pad the pipe clips with felt to quieten them, but this is a lot of work if done after boxing-in or flooring over.

You will also hear it after the dishwasher (very hot) or washing machine pumps out, or if you empty a kettle down the sink.