After further "investigations," I have the following suspicion:
The oil heating system (old unit) is in the basement and has a burner that can be heard. The chimney runs through the house up through the roof (not surprising) and the burner noise is conducted up inside the chimney. (I have been wearing a hearing aid for a few years and have noticed that it makes it difficult for me to locate noise sources. Although I hear reasonably well, I am not quite sure where the sound comes from.) Strangely, I have never noticed the noise on the middle floor until now and I do not notice it today either, but the upper part of the chimney was rebuilt – so it is made of different material than the old one.
I assume I hear the burner noise under the roof.
We plan to replace the oil heating system with a geothermal heating system. It has no burner and I therefore hope for a quiet retirement ... unless Putin comes all the way to North Hesse (this is meant to be funny, but it is by no means just funny :rolleyes:)