The hum of tinnitus does not necessarily affect the ability to hear - you can have excellent hearing acuity and still have tinnitus, which can manifest as constant or variable tones, from a very bass-toned hum to a high-pitched squeal. Some people even describe pulsing (like a drum) or birdsongs
You mentioned experiencing it in Copenhagen - if you leave the area and it persists, it is likely in you ear, not environmental.
Whoever discovers the exact cause of and cure for tinnitus will be a very popular, and probably very wealthy, person because it plagues millions of people. Mine has existed for nearly 40 years, and I have learned to largely ignore it.