Glad it stopped, Sibelius. Do you know what caused it? Did it change over the months?

No tapping, but the other night I got out of bed rhree times because every three minutes or so there was an electronic-sounding beep beep! At first I thought it was a low battery in my phone, so I disconnected it, both the phone cord and the electricity connection. Went back to bed in great satisfaction. And then beep beep! Got out of bed again and stood stock still. Ascertained it was not the smoke alarm, and not the other phone, which was not even connected in the first place. It sounded like it came from the floor. Moved two chairs, examined behind the drapes, checked the bedside table surface, checked my bedside clock. beep beep! Went back to bed. beep beep! Got ear plugs. Went back to bed.

Still have no idea what it was. Haven't heard it since.