None of my flatmates heard that sound, so I started questioning my perception until I found another person living in the same building hearing that sound. Our bedrooms are located one below the other and the sound is most perceivable in that rooms, but we can hear that sound at other places inside the building too while others can not.
Some time later, articles in various local newspapers about that hum appeared (search for "Brummen Hamburg" if you're interested, unfortunately only articles in german), telling stories about various people along the river "Elbe" hearing that hum. I started reading about the Windsor and Bristol hum that time too.
A local politician started investigating that issue together with scientist from a technical university but - besides some theories (e.g. a big power plant) which couldn't be proven - with no success. The acousticians where cited that some of them could hear the hum at various places too, but in terms of sound level they couldn't measure it distincly from the "general" background noises of the city (with harbour, industry and much highway traffic).
In the last few years there wasn't much news about it but the hum is still there in varying intensity and duration.